Defect reduction in seeded aluminum nitride crystal growth

ABSTRACT

Bulk single crystal of aluminum nitride (AlN) having an areal planar defect density≦100 cm −2 . Methods for growing single crystal aluminum nitride include melting an aluminum foil to uniformly wet a foundation with a layer of aluminum, the foundation forming a portion of an AlN seed holder, for an AlN seed to be used for the AlN growth. The holder may consist essentially of a substantially impervious backing plate.

RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 14/458,825, filed on Aug. 13, 2014, which is a continuation ofU.S. patent application Ser. No. 13/669,630, filed on Nov. 6, 2012, nowissued as U.S. Pat. No. 8,834,630, which is a continuation of U.S.patent application Ser. No. 12/015,957, filed on Jan. 17, 2008, nowissued as U.S. Pat. No. 8,323,406, which claims the benefit of andpriority to U.S. Provisional Application Ser. No. 60/880,869 filed Jan.17, 2007. This application is also a continuation-in-part of U.S. patentapplication Ser. No. 13/173,213, filed on Jun. 30, 2011, which claimsthe benefit of and priority to U.S. Provisional Patent Application No.61/360,142, filed Jun. 30, 2010. The disclosure of each of theseapplications is hereby incorporated by reference in its entirety.

GOVERNMENT SUPPORT

This invention was made with United States Government support undercontract number DE-FC26-08-NT01578 awarded by the Department of Energy(DOE) and contract number 70NANB4H3051 awarded by the National Instituteof Standards and Technology (NIST). The United States Government hascertain rights in the invention.

FIELD OF THE INVENTION

The present invention relates to the fabrication of single crystal AlN,and, more specifically, to the fabrication of single crystal AlN havinglow planar defect densities.

BACKGROUND

Aluminum nitride (AlN) holds great promise as a semiconductor materialfor numerous applications, e.g., opto-electronic devices such asshort-wavelength light-emitting diodes (LEDs) and lasers, dielectriclayers in optical storage media, electronic substrates, and chipcarriers where high thermal conductivity is essential, among manyothers. In principle, the properties of AlN may allow light emissiondown to around 200 nanometers (nm) wavelength to be achieved. The use ofAlN substrates is also expected to improve high-power radio-frequency(RF) devices, made with nitride semiconductors, due to the high thermalconductivity with low electrical conductivity. Addressing variouschallenges can help increase the commercial practicability of suchdevices.

For example, large-diameter bulk AlN crystals (grown, for example, usingthe techniques described in U.S. application Ser. No. 11/503,660,incorporated herein in its entirety, referred to hereinafter as the“'660 application”), may in some circumstances grow with hexagonal-prismshaped cavities defects that are roughly 0.5 millimeter (mm) in diameterand 0.1 mm thick. Area concentrations as high as 100 cm⁻² have beenobserved in AlN slices that are cut to be 0.5 mm thick from these largediameter boules. Similar kinds of defects have been observed in thegrowth of other hexagonal crystals, such as SiC, and are commonlyreferred to as planar defects. These defects may be problematic for thefurther development of nitride-based electronics. In particular, theytypically cause the surface of a substrate to roughen when theyintersect the surface plane. They may also scatter light, which may beproblematic for many opto-electronic applications that benefit from thetransparency of AlN substrates at optical wavelengths between 210 and4500 nm. Planar defects may also reduce the thermal conductivity aroundthe defect, an effect that is generally undesirable for high-powerdevices in which the high intrinsic thermal conductivity of the AlN isuseful. They may also introduce small angle grain boundaries into theAlN crystal and, thus, degrade the quality of the crystal by increasingthe effective concentration of dislocations that thread through from oneside of the wafer to the other (so-called threading dislocations) andthat degrade the quality of surface preparation. Thus, the applicationof AlN substrates to the fabrication of high-performance, high-poweropto-electronic and electronic devices may be enhanced if planar defectsare reduced or eliminated.

Generally, planar defect formation in crystals grown by physical vaportransport (PVT) is caused by voids that get trapped in the growingcrystal and that move and are shaped by the thermal gradients to whichthe crystal is exposed. A common cause identified in SiC crystal growthis poor seed attachment, where any kind of a microscopic void willcommonly result in the formation of a planar defect (see, e.g., T. A.Kuhr, E. K. Sanchez, M. Skowronski, W. M. Vetter and M. Dudley, J. Appl.Phys. 89, 4625 (2001) (2001); and Y. I. Khlebnikov, R. V. Drachev, C. A.Rhodes, D. I. Cherednichenko, I. I. Khlebnikov and T. S. Sudarshan, Mat.Res. Soc. Proc. Vol. 640, p. H5.1.1 (MRS 2001), both articles beingincorporated herein by reference in their entireties). In particular,poor seed attachment may cause voiding to occur between the seed andseed holder or may leave the back surface of the seed inadequatelyprotected, allowing AlN material to sublime from that surface. For AlNcrystal growth, crucible abnormalities, such as wall porosity or a seedmounting platform in which voids are present or can form, may also be acause of voiding.

A typical planar defect 10 is shown schematically in FIG. 1. In somecases the shape of the planar defects is not perfectly hexagonal butmodified or distorted and even triangular depending on the tilt betweenthe planar void and the c-plane {0001} of AlN. In addition, there istypically a small-angle grain boundary 20 in the trail of the planardefect as shown in the schematic diagram, the origin of which isdiscussed below. The planar defect has a height h₁, and leaves a planardefect trail of length h₂ that extends back to the origin of the planardefect, typically the back of the seed crystal.

FIGS. 2A and 2B show optical microscopy images of a 2-inch diameter,c-face (i.e., c-axis oriented parallel to the surface normal of thewafer) AlN substrate taken after fine mechanical polishing. Theright-side image (FIG. 2B) represents the same location as in theleft-side image (FIG. 2A) taken in cross-sectioned analyzer-polarizer(AP) mode. The planar defect dimensions vary from 0.1 to 2 mm in widthand up to 0.5 mm in depth, although they generally tend to be thinner(˜0.1 mm) However, the base of the planar defect is typicallymisoriented with respect to the overall crystal (typically a smallrotation about the c-axis), and thus there is a boundary between theoriginal crystal and the slightly misoriented material that is below theplanar defect. This boundary is defined by dislocations that account forthe misorientation of the material below the planar defect.

Causes of Planar Defects

If the AlN seed is poorly attached in a way that allows material in theback of the seed to move under the temperature gradient, then thismaterial movement may cause voids to “enter” the seed. This effect isdue to the fact that every void has a small but defined axial gradientthat drives material to be evaporated and then re-condensed within thevoid. The voids entering the AlN bulk material form well-definedhexagonal-prism shapes, probably because of the anisotropy in surfaceenergy formation.

Migration of the Planar Defect in a Thermal Gradient and ResultingDegradation of Crystal

Referring to FIGS. 3A and 3B, growth inside planar defects has beendocumented. The growth facet in FIG. 3B is pronounced, indicatingfaceted growth mode within the planar defect. Faceted growth modeusually results in a high-quality crystal. It can be expected,therefore, that the material quality within the planar defect is highand may be dislocation-free.

As the crystal grows, the planar defects effectively migrate toward thegrowth interface due to the axial temperature gradient within the void.Planar defects travel from the seed toward the growth interface becauseof the axial gradient across the planar height. As a result of thismovement, the planar defects may leave “trails” (or imprints) of grainboundaries with very small misorientation angles. These small-anglegrain boundaries are pronounced and shaped according to the planardefect symmetry. An example of this is shown in FIG. 4 and discussedbelow.

According to the traditional Read's model of low-angle grain boundaries,a boundary typically contains pure edge dislocations lying in the planeof the boundary. Therefore, after etching, the boundary is expected toexhibit a number of separated etch pits. The greater the distancebetween the pits, the smaller will be the misorientation angle. Thegrain boundary angle may be found using Frank's formula:

$\begin{matrix}{{\frac{b}{D} = {2\mspace{14mu} {\sin \left( \frac{\theta}{2} \right)}}},} & (1)\end{matrix}$

where D is the distance between dislocations (etch pits), b is theBurgers vector of dislocation, and θ is the misorientation angle. InFIG. 4, the closest distance between the etch pits is ˜12 micrometers(μm), and the Burgers vector for pure edge dislocation perpendicular tothe [0001] planes is equal to the “a” lattice constant, i.e. 0.3111 nmTherefore, the azimuthal misorientation angle of the planar defect wallsis expected to be about 0.0004° (or 1.44 arcsec).

Thus, in addition to the problems caused by the physical presence ofplanar defects, the formation and motion of planar defects in thecrystal during growth may also degrade the overall crystal quality. Thisdegradation results because of the slight misorientation between theplanar defect body and the AlN bulk material. As the planar defect movesthrough the crystal, it leaves behind a grain boundary, as shown inFIG. 1. These grain boundaries typically show misorientation of about 2arcseconds for individual planar defects. However, if the density of theplanar defects is high, each of these randomly misoriented grainboundaries can add up and result in much higher “effective”misorientation and, as a result, much lower crystal quality. Analternative way to look at the degradation of crystal quality is toconsider the increase in threading dislocation density due to the planardefects. As one may calculate from the micrograph shown in FIG. 4, eachplanar defect may create over 10⁴ dislocations/cm² in its wake.

Problems with Surface Preparation Due to Planar Defects

Planar defects may affect preparation and polishing of AlN wafers. Thesharp edges of planar defects intersecting the AlN sample surface maychip off and cause scratching. In addition, planar defects—being relatedto the small-angle grain boundaries (SAGB)—may result in surfaceroughening (topography) during chemical-mechanical polishing (CMP)treatment.

FIGS. 5A and 5B respectively show the surface and the bulk depth of AlNcontaining planar defects and LAGB, where the images are obtained at thesame location. It is clear that the planar defects and the SAGB causesurface roughening which, in turn, affects the epitaxial growth.

Problems with Optical Transparency and Thermal Conductivity

Planar defects may have a negative impact on the optical-transmissionproperties of AlN wafers because they scatter light due to theintroduction of additional interfaces within the crystal, which separateregions with different refractive indices. In addition, while AlNsubstrates are attractive because of their high thermal conductivity(which can exceed 280 W/m-K at room temperature), planar defects maycause the thermal conductivity to diminish in a location directly abovethe planar defect because of the extra interfaces that are inserted atthe planar defect boundaries as well as the thermal resistance of thevolume of the planar defect itself. This local increase of the thermalresistance of the AlN substrate may reduce the usefulness of the AlNsubstrates for applications that require high power dissipation, e.g.,high-power RF amplifiers and high-power, high-brightness LEDs and laserdiodes.

Limitations of Existing Methods

As described in the '660 application, the production of large-diameter(i.e., greater than 20 mm) AlN crystals typically requires seededgrowth. However, as discussed below, the seed holder and seed mountingtechnique on the holder are primary sources of planar defects in the AlNboules that are produced. The '660 application discloses a method forAlN seed attachment and subsequent crystal growth. Referring to FIG. 6,an AlN ceramic-based, high-temperature adhesive bonds the AlN seed tothe holder plate and at the same time protects the back of the AlN seedfrom sublimation. In particular, an AlN seed 100 is mounted onto aholder plate 130 using an AlN-based adhesive 140. The AlN ceramicadhesive may contain at least 75% AlN ceramic and silicate solution thatprovides adhesive properties. One suitable example of such an adhesiveis Ceramabond-865 available from Aremco Products, Inc.

In a particular version, the AlN seed is mounted using the followingprocedure:

(1) AlN adhesive is mixed and applied to the holder plate using a brushto a thickness not exceeding about 0.2 mm;

(2) The AlN seed is placed on the adhesive; and then

(3) The holder plate along with the seed is placed in a vacuum chamberfor about 12 hours and then heated up to 95° C. for about 2 hours.

This approach has proven successful in providing high-quality,large-diameter AlN crystal boules. However, planar defects as shown inFIGS. 2A and 2B will form. This problem is caused by the voids leftbehind as the silicate solution is either evaporated or absorbed by theAlN seed crystal or by Al escaping through the seed holder.

An alternative method for AlN seed attachment and subsequent crystalgrowth described in the '660 application involves mounting the AlN seedon a thin foil of Al on the holder plate. The Al is melted as thetemperature of the furnace is raised above 660° C. (the melting point ofAl), thereby wetting the back of the seed and the holder plate. As thetemperature is raised further, the Al reacts with N₂ in the furnace toform AlN, which secures the seed to the holder plate. This technique mayrequire that the AlN seed be held in place (either by gravity ormechanically) until a sufficient amount of the Al has reacted to formAlN, after which no further mechanical support is needed.

This technique, too, results in planar defects. The Al foil may melt andball up, leaving empty spaces between agglomerations of liquid Al. Theagglomerated liquid-Al metal may then react to form a nitride, leavingempty spaces between the seed and the seed holder. These empty spaces,in turn, can lead to planar defects once crystal growth is initiated onthe seed crystal. The interaction between the AlN seed and the seedholder may also contribute to defects. Typically some amount ofdiffusion of either Al or N (or both) into the seed holder occurs at thetemperatures used for crystal growth. For instance, a tungsten (W) seedholder may absorb both Al and N at the growth temperature, which canresult in planar defects forming in the seed crystal and in theresulting boule grown from the seed crystal. In addition, the seedholder may have a thermal expansion coefficient different from that ofthe AlN crystal, which may cause defects in the seeded crystal or mayinduce voids to open up at the seed crystal/seed holder interface,resulting in planar defects during subsequent boule growth.

Another way to attach the seed crystal to the seed holder is to run aheat cycle under conditions whereby the seed is held onto the seedbacking (e.g., by placing the seed crystal under an appropriate massthat holds the crystal down during this process), and heating thecrystal up to a temperature above 1800° C. (and preferably above 2000°C.) to allow the seed to thermally, chemically and/or mechanically bondto the seed holder material. This approach is referred to herein assinter bonding. The sintering process may, however, be difficult tocontrol such that good bonding occurs without damaging the seed. Inaddition, it may be difficult to avoid leaving some space between theseed crystal and the seed holder. This space may be filled duringprocessing with AlN that mostly comes from the seed crystal (even whenvapors of Al and N₂ are supplied by having an AlN ceramic present in thecrucible during the sintering process), and this AlN may induce planardefects to form in the seed crystal that may propagate into thesingle-crystal boule grown on the seed crystal.

To make large-diameter AlN substrates more readily available andcost-effective, and to make the devices built thereon commerciallyfeasible, it is also desirable to grow large-diameter (>25 mm) AlN bulkcrystals at a high growth rate (>0.5 mm/hr) while preserving crystalquality. As mentioned above, the most effective method of growing AlNbulk single crystals is the “sublimation-recondensation” method thatinvolves sublimation of lower-quality (typically polycrystalline) AlNsource material and recondensation of the resulting vapor to form thesingle-crystal AlN. U.S. Pat. No. 6,770,135 (the '135 patent), U.S. Pat.No. 7,638,346 (the '346 patent), and U.S. Pat. No. 7,776,153 (the '153patent), the entire disclosures of which are incorporated by referenceherein, describe various aspects of sublimation-recondensation growth ofAlN, both seeded and unseeded. While these references recognize thebenefits of a large axial (i.e., parallel to the primary growthdirection) thermal gradient for optimizing material quality and growthrate of the growing AlN crystal, they utilize a growth apparatusdesigned to minimize the radial (i.e., perpendicular to the primarygrowth direction) thermal gradient. For example, axial thermal gradientsmay range from approximately 5° C./cm to approximately 100° C./cm, whileradial thermal gradients are maintained at as negligible a level aspossible. Likewise, other prior-art growth apparatuses utilize heavyinsulation in order to minimize or eliminate the radial thermalgradient, as a minimized radial thermal gradient is expected to produceflat, high-quality crystals, particularly when efforts are made to growcrystals having large diameters. The radial gradient is typicallyminimized during conventional crystal growth in order to preventformation of defects such as dislocations and low-angle grainboundaries. It is also minimized to make the surface of the growingcrystal more flat, thus increasing the amount of useable material in thecrystal (i.e., increasing the number of substrates that can be cut fromthe crystal for a given length of crystal).

FIG. 7 depicts an apparatus 700 utilized for the growth of AlN inaccordance with the above-described prior art. As shown, a crucible 705is positioned on top of a crucible stand 710 within a cylindricalsusceptor 715. During the growth process, the susceptor 715 istranslated within a heated zone created by surrounding heating coils(not shown), polycrystalline AlN source material 720 at the base 725 ofthe crucible sublimes at the elevated temperature, and the resultingvapor recondenses at the cooler tip 730 of the crucible due to the largeaxial thermal gradient between the base 725 and the tip 730, thusforming an AlN crystal 735. The apparatus 700 also features top axialshields 740 and bottom axial shields 745 designed and positioned tominimize the radial thermal gradient perpendicular to the growthdirection 750 of AlN crystal 735. As shown, the tip 730 of the crucible705 is cooler than the base 725 at least in part because apparatus 700has fewer top axial shields 740 than bottom axial shields 745, allowingmore heat to escape in the region of tip 730 and generating the desiredaxial thermal gradient. The top axial shields 740 may have centeredholes therewithin to facilitate measurement of the temperature at tip730 by a pyrometer 755. The centered hole diameter is minimized toreduce the heat flow but sufficient to form a practical optical path forthe temperature sampling by the pyrometer 755. Additional pyrometers760, 765 may also be utilized to measure temperatures at other regionsof apparatus 700.

The ability to grow AlN single crystals at high growth rates would spuradditional commercial adoption of the technology. While increasing thegrowth rate of AlN crystals is theoretically possible by increasing theAl supersaturation using larger axial thermal gradients, increases inthe Al supersaturation may result in deterioration of the materialquality of the crystal, or even in polycrystalline, rather thansingle-crystal, growth. Furthermore, the minimization or elimination ofradial thermal gradients during AlN crystal growth unexpectedly tends todeleteriously impact the quality of the AlN crystal, particularly whenattempts are made to grow large (e.g., >25 mm diameter) crystals atreasonable growth rates (e.g., >0.5 mm/hr). Thus, a need exists forsystems and techniques enabling growth of such large AlN crystals athigh growth rates while still preserving high material quality of theAlN crystal.

SUMMARY

Embodiments of the invention allow the reduction or elimination ofplanar defects during the growth of bulk aluminum nitride (AlN)crystals, i.e., boules. In particular, in some embodiments, the arealplanar defect density is reduced to less than 100/cm² and, preferably,to less than 1/cm². As a result, the fabrication of crystalline AlNwafers larger than 20 mm in diameter with a thickness ranging from 0.1-1mm and having planar defect density of less than 1 cm⁻² is enabled.

Key factors that enable growing seeded, large diameter, high quality AlNcrystals include:

1.) The seed crystal itself is free of planar defects, as well as freefrom other kinds of defects that may form voids (that generally evolveinto planar defects at the crystal growth temperature). A defect to beconsidered is subsurface damage that may be introduced into the seedcrystal by the cutting and polishing process.

2.) The seed is attached to a seed holder (with seed holder beingdefined in the '660 application and described in detail above withreference to FIG. 6) in such a way as to prevent the formation of voidsbetween the seed and the seed holder This may be accomplished byproperly preparing the back surface of the seed (also referred to as themounting surface of the seed, as opposed to the front surface of theseed that is used to seed the bulk crystal growth) as well as thesurface of the seed holder. A film is then applied to the back of theseed, conforming microscopically with both the back surface of the seedas well as the seed holder. This film is preferably completely dense(i.e., no microscopic voids).

3.) The seed holder is relatively impervious to aluminum transport so asnot to form voids in growing AlN crystal. In some examples, the filmused to attach the seed to the seed holder is, itself, impervious toaluminum transport. In some of the implementations described below, theseed holder is only impervious to aluminum transport for a certainperiod of time. This time limitation generally limits the length of theAlN boule that can be grown.

Since a principal way the AlN crystal compensates for the diffusion ofAl out of the crystal is by the formation of planar defects, the maximumallowable rate for transport through the seed holder assembly may beestimated for a given planar defect density. For instance, to keep thedensity of planar defects below 100/cm², the maximum allowable number ofatoms of Al that may be allowed to diffuse through the seed holderassembly is generally <10²⁰/cm² over the period of time that the AlNcrystal is being grown. To keep the planar defect density below 1/cm²,the Al diffusion is preferably kept below 10¹⁸ atoms of Al per cm².

4.) Stress between the seed holder assembly and the seed is reduced.This may be achieved by (i) either the thermal expansion of the seedholder assembly nearly matching the thermal expansion of the AlN seed inthe temperature range from room temperature up to the growth temperature(2200° C.), or (ii) the seed holder assembly being sufficientlymechanically flexible to absorb the thermal expansion mismatch throughdeformation while reducing the strain on the seed crystal and theresulting AlN boule. This factor does not typically allow theachievement of the third factor above by simply making the seed holderthicker.

5.) Generally, it is also desirable for the seed holder assembly to haveenough mechanical strength to be able to support the growing AlN boulewhile, at the same time, providing a sealing surface to the crucibleused to contain the AlN material and Al vapor (as described in the '660application). However, the mechanical strength needed typically dependson the crystal growth geometry used. Less mechanical strength may beneeded if the seed crystal is placed at the bottom of the crystal growthcrucible; this geometry, however, may need tighter control of the AlNsource material to prevent particles falling from the source materialnucleating defects in the growing crystal.

Moreover, conditions for high quality AlN crystal growth are preferablyfollowed, as described in the '660 application. In particular,super-atmospheric pressures may be successfully utilized to producesingle crystals of AlN at relatively high growth rates and crystalquality. To achieve this, one or more of the following may becontrolled: (i) temperature difference between an AlN source materialand growing crystal surface, (ii) distance between the source materialand the growing crystal surface, and (iii) ratio of N₂ to Al partialvapor pressures. Increasing the N₂ pressure beyond the stoichiometricpressure may force the crystal to grow at a relatively high rate due tothe increased reaction rate at the interface between the growing crystaland the vapor. This increase in the growth rate has been shown tocontinue with increasing N₂ partial pressure until diffusion of Al fromthe source to the growing crystal (i.e., the negative effects ofrequiring the Al species to diffuse through the N₂ gas) becomes therate-limiting step. Employing higher-pressure nitrogen may have theadded benefit of reducing the partial pressure of aluminum inside thegrowth crucible, which may decrease corrosion within the furnace oftencaused by Al vapor inadvertently escaping the crucible. To growhigh-quality AlN crystals, very high temperatures, for example exceeding2100° C., are generally desirable. At the same time, high thermalgradients are needed to provide sufficient mass transport from thesource material to the seed crystal. If not chosen properly, thesegrowth conditions may result in evaporation of seed material or itstotal destruction and loss. The AlN seeded bulk crystal growth may becarried out in a tungsten crucible using a high-purity AlN source. Thetungsten crucible is placed into an inductively heated furnace so thatthe temperature gradient between the source and the seed material drivesvapor species to move from hotter high purity AlN ceramic source to thecooler seed crystal. The temperature at the seed interface and thetemperature gradients are monitored and carefully adjusted, ifnecessary, in order to nucleate high-quality mono-crystalline materialon the seed and not destroy the AlN seed.

Hereinafter, several ways to implement these concepts are described indetail, and specific examples of implementation are provided.

Embodiments of the current invention also achieve high growth rates(e.g., greater than approximately 0.5 mm/hr) of large, high-qualitysingle-crystal semiconductors (e.g., AlN) by forming and maintainingnon-zero axial and radial thermal gradients in the growth apparatus suchthat the ratio of the axial thermal gradient to the radial thermalgradient (the “thermal gradient ratio”) is greater than zero and lessthan 10. (As utilized herein, a thermal gradient being maintained doesnot necessarily imply that it is held constant as a function of time,only that it is non-zero (and constant or fluctuating) over a period oftime.) The size and the quality of growing crystals are generallyinfluenced by the thermal field within the growth cell. The axialthermal gradient is the magnitude of the thermal field projected on thelongitudinal symmetry axis in a cylindrical coordinate system. Theradial thermal gradient is the projection of the thermal field magnitudeon the azimuthal direction. Therefore, the thermal gradient in any otherdirection may be described as a superposition of the axial and radialthermal gradients (and thus may also be controlled as the axial and/orradial thermal gradients are controlled). The deliberate formation andcontrol of the radial thermal gradient large enough to result in athermal gradient ratio less than 10 contradicts the above-describedconventional wisdom in which radial thermal gradients (which may dependat least in part on the dimensions and shape of the growth chamber),even if formed at all (e.g., unintentionally) are eliminated orminimized to small magnitudes.

In some embodiments, the radial thermal gradient and the axial thermalgradient are substantially balanced and, preferably, the thermalgradient ratio ranges from approximately 1.2 to approximately 5.5. Inorder to facilitate formation and control of the radial thermalgradients, crystal-growth apparatuses in accordance with variousembodiments of the invention utilize different types, thicknesses,and/or arrangements of thermal shields, particularly in the area“behind” the growing crystal (i.e., corresponding to the location of thetop axial shields 740 in FIG. 7). Thus, for embodiments featuring seededgrowth of AlN single crystals, one or more shields are typically locatedopposite the growth surface of the seed. The one or more shieldsutilized in preferred embodiments of the invention include or consistessentially of one or more refractory materials, e.g., tungsten, and maybe substantially thin, i.e., have thicknesses less than 0.5 mm, e.g.,ranging from 0.125 mm to 0.5 mm.

In an aspect, embodiments of the invention may include a bulk singlecrystal of AlN having a diameter greater than 20 mm, a thickness greaterthan 0.1 mm, and an areal planar defect density may be less than orequal to 100 cm⁻².

One or more of the following features may be included. The areal planardefect density may be measured by counting all planar defects in thebulk single crystal and dividing by a cross-sectional area of the bulksingle crystal disposed in a plane perpendicular to a growth directionthereof. The bulk single crystal may be in the form of a boule having athickness greater than 5 mm. The areal planar defect density may be lessthan or equal to 1 cm⁻².

The single crystal AlN may be in the form of a wafer. The areal planardefect density may be less than or equal to 10 cm⁻². An areal planardefect density of planar defects intersecting each of a top and a bottomsurface of the wafer may be less than or equal to 1 cm⁻².

In another aspect, embodiments of the invention may include a bouleincluding a bulk single crystal of AlN having a diameter greater than 20mm, a thickness greater than 5 mm, and an areal density of threadingdislocations of less than or equal to 10⁶ cm⁻² in each cross-section ofthe bulk single crystal disposed in a plane perpendicular to a growthdirection of the crystal. In some embodiments, the areal density ofthreading dislocations may be less than or equal to 10⁴ cm⁻².

In yet another aspect, embodiments of the invention feature a bouleincludes a bulk single crystal of AlN having a sufficient thickness toenable the formation of at least five wafers therefrom, each waferhaving a thickness of at least 0.1 mm, a diameter of at least 20 mm, anda threading dislocation density of less than or equal to 10⁶ cm⁻². Insome embodiments, each wafer may have a threading dislocation densityless than or equal to 10⁴ cm⁻².

In still another aspect, embodiments of the invention include a bouleincluding a substantially cylindrical bulk single crystal of MN having adiameter of at least 20 mm and having a sufficient thickness to enablethe formation of at least five wafers therefrom, each wafer having athickness of at least 0.1 mm, a diameter of at least 20 mm, and atriple-crystal X-ray rocking curve of less than 50 arcsec full width athalf maximum (FWHM) for a (0002) reflection. Each wafer hassubstantially the same diameter as each of the other wafers.

In another aspect, embodiments of the invention include a method forgrowing single-crystal aluminum nitride (AlN). The method includesproviding a holder including a backing plate, the holder (i) being sizedand shaped to receive an AlN seed therein and (ii) including an AlNfoundation bonded to the backing plate. An Al foil is interposed betweenthe seed and the AlN foundation. The Al foil is melted to uniformly wetthe foundation with a layer of Al. An AlN seed is disposed within theholder. Aluminum and nitrogen are deposited onto the seed underconditions suitable for growing single-crystal AlN originating at theseed.

One or more of the following features may be included. The back platemay be conditioned to reduce permeability of the back plate to Al. Theseed crystal may be a wafer having a diameter of at least 20 mm. Thegrown single-crystal AlN may define a boule having a diameterapproximately the same as a diameter of the seed crystal.

In another aspect, embodiments of the invention feature a method forgrowing single-crystal aluminum nitride (AlN). The method includesproviding a holder sized and shaped to receive an AlN seed therein, theholder consisting essentially of a substantially impervious backingplate. An AlN seed is disposed within the holder. An Al foil isinterposed between the seed and the backing plate. The Al foil is meltedto uniformly wet the backing plate and the back of the AlN seed with alayer of Al. Aluminum and nitrogen are deposited onto the seed underconditions suitable for growing single-crystal AlN originating at theseed.

In another aspect, embodiments of the invention feature a method offorming single-crystal aluminum nitride (AlN). Vapor comprising orconsisting essentially of aluminum and nitrogen is condensed within agrowth chamber, thereby forming an AlN single crystal that increases insize along a growth direction. During the formation, a first (e.g.,axial) non-zero thermal gradient is formed and maintained within thegrowth chamber in a direction substantially parallel to the growthdirection, and a second (e.g., radial) non-zero thermal gradient isformed and maintained within the growth chamber in a directionsubstantially perpendicular to the growth direction. The ratio of thefirst thermal gradient to the second thermal gradient is less than 10.

Embodiments of the invention may include one or more of the following inany of a variety of combinations. Solid source material (which mayinclude or consist essentially of, e.g., polycrystalline AlN) may besublimed within the growth chamber to form the vapor. The second thermalgradient may be larger than 4° C./cm and/or smaller than 85° C./cm. Theratio of the first thermal gradient to the second thermal gradient maybe greater than 1.2. The first thermal gradient may be larger than 5°C./cm and/or smaller than 100° C./cm. The ratio of the first thermalgradient to the second thermal gradient may be less than 5.5, or evenless than 3.

Forming the second thermal gradient may include or consist essentiallyof arranging a plurality of thermal shields outside the growth chamber.Each of the thermal shields may include or consist essentially of arefractory material, e.g., tungsten. Each thermal shield may define anopening therethrough. The openings of the thermal shields may besubstantially equal in size to each other. The opening of each thermalshield may range from approximately 10 mm to approximately 2 mm lessthan the dimension of the growth chamber substantially perpendicular tothe growth direction. The openings of at least two of the thermalshields may be different in size. A first thermal shield having a firstopening may be disposed between the growth chamber and a second thermalshield, the second thermal shield having a second opening larger thanthe first opening. At least two of the thermal shields may havedifferent thicknesses. The thickness of each of the thermal shields mayrange from approximately 0.125 mm to approximately 0.5 mm.

The growth chamber may include a lid disposed between the AlN singlecrystal and at least one (or even all) of the thermal shields. Thethickness of the lid may be less than approximately 0.5 mm. The lid mayinclude or consist essentially of tungsten. A seed may be disposedwithin the growth chamber before forming the AlN single crystal, and theAlN single crystal may form on the seed and extend therefrom in thegrowth direction. The diameter of the seed may be greater thanapproximately 25 mm. The growth rate of the AlN single crystal may begreater than approximately 0.5 mm/hr. The AlN single crystal may form ona seed disposed within the growth chamber.

In another aspect, embodiments of the invention feature a crystal-growthsystem including or consisting essentially of a growth chamber for theformation of a single-crystal semiconductor material viasublimation-recondensation therewithin along a growth direction, aheating apparatus for heating the growth chamber, and a plurality ofthermal shields arranged to form, within the growth chamber, (i) a firstnon-zero thermal gradient in a direction substantially parallel to thegrowth direction and (ii) a second non-zero thermal gradient in adirection substantially perpendicular to the growth direction. The ratioof the first thermal gradient to the second thermal gradient is lessthan 10.

Embodiments of the invention may include one or more of the following inany of a variety of combinations. Each thermal shield may define anopening therethrough. The openings of at least two of the thermalshields may be different in size. A first thermal shield having a firstopening may be disposed between the growth chamber and a second thermalshield, the second thermal shield having a second opening larger thanthe first opening. At least two of the thermal shields may havedifferent thicknesses. The thickness of each of the thermal shields mayrange from approximately 0.125 mm to approximately 0.5 mm Each of thethermal shields may include or consist essentially of a refractorymaterial, e.g., tungsten. The thermal shields may be arranged withsubstantially equal spacings therebetween. A seed for nucleating thesingle-crystal semiconductor material thereon may be disposed within thegrowth chamber. The diameter of the seed may be greater thanapproximately 25 mm, and/or the seed may include or consist essentiallyof aluminum nitride. The ratio of the first thermal gradient to thesecond thermal gradient may be less than 5.5, or even less than 3. Theratio of the first thermal gradient to the second thermal gradient maybe greater than 1.2.

In yet another aspect, embodiments of the invention feature a method forgrowing single-crystal aluminum nitride (AlN). An AlN seed (i.e., a seedcrystal including, consisting essentially of, or consisting of AlN) ismounted on a seed holder, thereby forming a seed-seed holder assembly.The seed-seed holder assembly is disposed within a crystal-growthcrucible. The crystal-growth crucible is heated to apply thereto (i) aradial thermal gradient of less than 50° C./cm and (ii) a verticalthermal gradient greater than 1° C./cm and less than 50° C./cm. Aluminumand nitrogen are deposited onto the AlN seed under conditions suitablefor growing single-crystal AlN originating at the AlN seed.

Embodiments of the invention may include one or more of the following inany of a variety of combinations. AlN source material may be disposedwithin the crystal-growth crucible. The deposited aluminum and nitrogenmay evolve from the AlN source material during heating of thecrystal-growth crucible. The AlN source material may be at leastpartially polycrystalline. The seed-seed holder assembly may be affixedto a lid of the crystal-growth crucible. Mounting the AlN seed on theseed holder may include or consist essentially of disposing a foilbetween the AlN seed and the seed holder. The foil may be substantiallyimpervious to aluminum transport and/or substantially impervious tonitrogen. The foil may include, consist essentially of, or consist oftungsten. The foil may include, consist essentially of, or consist ofsingle-crystalline tungsten. The foil may include, consist essentiallyof, or consist of aluminum. The seed holder may be substantiallyimpervious to aluminum transport and/or substantially impervious tonitrogen. A barrier layer may be disposed over at least a portion of asurface of the AlN seed. The barrier layer may include, consistessentially of, or consist of tungsten, Hf, HfN, HfC, W—Re, W—Mo, BN,Ta, TaC, TaN, Ta₂N, and/or carbon. The barrier layer may include,consist essentially of, or consist of tungsten. The AlN seed mayinclude, consist essentially of, or consist of a wafer having a diameteror width of at least 20 mm. The grown single-crystal AlN may have adiameter (or width) greater than 20 mm, a thickness greater than 0.1 mm,and/or an areal planar defect density≦100 cm⁻². The areal planar defectdensity may be ≦1 cm⁻². Any gap between the AlN seed and the seed holdermay be minimized or substantially eliminated by positioning a weight onthe seed-seed holder assembly. The weight may be positioned on the AlNseed. The weight may include, consist essentially of, or consist oftungsten. The weight may be removed from the seed-seed holder assemblyprior to depositing aluminum and nitrogen onto the AlN seed.

The ratio of the vertical thermal gradient to the radial thermalgradient may be less than 10. The ratio of the vertical thermal gradientto the radial thermal gradient may be less than 5.5. The ratio of thevertical thermal gradient to the radial thermal gradient may be lessthan 3. The ratio of the vertical thermal gradient to the radial thermalgradient may be greater than 1.2. The radial thermal gradient may belarger than 4° C./cm. The vertical thermal gradient may be larger than5° C./cm. Applying the radial thermal gradient may include, consistessentially of, or consist of arranging a plurality of thermal shieldsoutside the crystal-growth crucible. One or more (even all) of thethermal shields may include, consist essentially of, or consist of arefractory material. One or more (even all) of the thermal shields mayinclude, consist essentially of, or consist of tungsten. One or more(even all) of the thermal shields may define an opening therethrough.The openings of the thermal shields may be substantially equal in sizeto each other. The openings of one or more (even all) of the thermalshields may range from approximately 10 mm to approximately 2 mm lessthan a dimension of the growth chamber substantially perpendicular to agrowth direction along which the single-crystal AlN grows. The openingsof at least two (even all) of the thermal shields may be different insize. A first thermal shield having a first opening may be disposedbetween the crucible and a second thermal shield, the second thermalshield having a second opening larger than the first opening. At leasttwo (even all) of the thermal shields may have different thicknesses.The thickness of each of the thermal shields may range fromapproximately 0.125 mm to approximately 0.5 mm.

These and other objects, along with advantages and features of theinvention, will become more apparent through reference to the followingdescription, the accompanying drawings, and the claims. Furthermore, itis to be understood that the features of the various embodimentsdescribed herein are not mutually exclusive and can exist in variouscombinations and permutations. Unless otherwise indicated, “radial”generally refers to a direction substantially perpendicular to theprimary crystal growth direction and/or the long axis of the crystaland/or the crystal-growth apparatus. Refractory materials are generallymaterials that are physically and chemically stable at temperaturesabove approximately 500° C. As used herein, the term “substantially”means±10%, and, in some embodiments, ±5%. The term “consists essentiallyof” means excluding other materials that contribute to function, unlessotherwise defined herein. Nonetheless, such other materials may bepresent, collectively or individually, in trace amounts.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, like reference characters generally refer to the sameparts throughout the different views. Also, the drawings are notnecessarily to scale, emphasis instead generally being placed uponillustrating the principles of the invention. In the followingdescription, various embodiments of the present invention are describedwith reference to the following drawings, in which:

FIG. 1 is a schematic diagram showing an idealized planar defect thattypically appears as a hexagonal void in an AlN crystal;

FIGS. 2A and 2B are optical micrographs of AlN single crystal samplecontaining planar defects: A) an optical image showing one planardefect, and B) image taken from the same location in birefringencecontrast showing multiple planar defects just underneath the surface;

FIGS. 3A and 3B are micrographs demonstrating growth features insideplanar defects, that are due to the movement of the planar defectsduring the crystal growth, with FIG. 3A being an optical micrograph, and3B being a micrograph taken in Nomarski Differential Image Contrast(NDIC);

FIG. 4 is an NDIC micrograph of the planar defect wake and grainboundaries marked by an etch pit associated with dislocations;

FIGS. 5A and 5B are micrographs illustrating the effect of the low-anglegrain boundaries due to planar defects on the surface finish, with FIG.5A taken after the CMP process, and 5B being a birefringence contrastimage from the same location showing planar defects;

FIG. 6 is a schematic diagram of an AlN seed mounting technique usinghigh temperature AlN ceramic-based adhesive;

FIG. 7 is a schematic cross-section of a crystal-growth apparatus inaccordance with the prior art in which radial thermal gradients areminimized or eliminated;

FIG. 8A is a graph illustrating the axial distribution (along the growthaxis) of the density of planar defects intersecting the surface of aslice through the boule: wafer #1 is closest to the growth interfacewhile wafer #4 is closest to the seed;

FIG. 8B is a schematic diagram illustrating the bonding of an AlN seedto a seed holder, (which, in a preferred implementation, includes,consists essentially of, or consists of an AlN foundation on a W backingplate);

FIG. 9 is a schematic diagram illustrating the technique for bonding anAlN seed crystal to a seed holder, which, in a preferred implementation,uses Al-foil nitridation;

FIG. 10 is a schematic diagram illustrating an assembled crystal growthcrucible;

FIG. 11 is a schematic cross-section of a crystal-growth apparatus inaccordance with various embodiments of the invention in which radialthermal gradients are generated and/or controlled;

FIGS. 12A and 12B are schematic cross-sections of portions of thecrystal-growth apparatus of FIG. 11 with alternate arrangements of topthermal shields, in accordance with various embodiments of theinvention; and

FIG. 13 is a table listing various steps of a process for growth ofsemiconductor crystals such as AlN in accordance with variousembodiments of the invention.

DETAILED DESCRIPTION

In accordance with some embodiments of the invention, one or more of themeasures described below may be taken to reduce defect generation duringseeded AlN growth of, e.g., boules.

As used herein, boule means an as-grown crystal of AlN that haspredominately (more than 50%) a single orientation. To betechnologically useful, the boule is preferably at least 20 mm indiameter and more than 5 mm in length, and the orientation preferablyvaries by no more than 1.5° across the width of the boule.

As used herein, wafer means a slice of AlN cut from a boule. Typically,a wafer has a thickness of between 0.1 mm to 1 mm and a diameter greaterthan 20 mm. However, wafers thinner than 0.1 mm, while fragile, may betechnologically useful for some specialized applications (for instance,in an application where optical transmission through the wafer iscritical).

High quality bulk single crystal AlN having low planar defect densities,and methods for formation thereof, are disclosed herein. Referring againto FIG. 4, each planar defect may create over 10⁴ dislocations/cm² inits wake. Thus, to prepare AlN wafers (which are thin slabs, typically0.1 to 1 mm thick, cut from the bulk crystal) with threading dislocationdensities (TDD) below 10⁶/cm², the areal planar defect density (definedas the number of planar defects that have passed through a unit area inthe bulk crystal) is generally kept below 100/cm² or below 1/cm² if theTDD is to be kept below 10⁴/cm². An areal planar defect density may bemeasured by counting all planar defects in a bulk single crystal anddividing by a cross-sectional area of the bulk single crystal disposedin a plane perpendicular to a growth direction thereof. Because thetemperature gradient along the crystal height increases toward the seed,so one may generally expect that the density of planar defects woulddecrease toward the growth interface (crown).

An illustration of this effect in AlN boules is shown in FIG. 8A, thatshows the axial distribution (along the growth axis) of the density ofplanar defects that intersect the surface of the wafer, with wafer #1being closest to the growth interface (crown) while wafer #4 is closestto the seed. The planar defects that intersect the surface of wafer #1have passed through the region of the boule represented by wafer #4,which may be observed by etching their trails as shown in FIG. 4. Sinceit may be difficult to see all the planar defects in a thick boule, theareal density may be measured by cutting a thin slice (0.1 to 0.8 mmthick) from the boule perpendicular to the growth direction andpolishing both surfaces of the slice with any anisotropic etch. Theareal planar defect density may then be estimated by totaling the numberof planar defects observed in the slice (both on the surface and underthe surface) and the number of planar defect trails that are observed onthe surface of the slice due to the preferential etching of defects, andthen dividing by the area of the slice. As FIG. 8A shows, the arealdensity of planar defects measured in a slice near the original seedcrystal will generally be higher than the areal density measured nearthe crown. Thus, to get the true areal planar defect density (and, thus,determine the number of low defect wafers that can be sliced from theboule) a slice from the boule is preferably selected from near the seedside of the boule. The areal densities of planar defects in wafers orseed plates sliced from a boule may be measured in the same way.High-resolution x-ray diffraction (XRD) rocking curves are a commonlyused indication of the crystal quality and may be used to estimate thedislocation density. See Lee et al., “Effect of threading dislocationson the Bragg peakwidths of GaN, AlGaN, and AlN heterolayers,” Appl.Phys. Lett. 86, 241904 (2005), incorporated by reference in itsentirety. Based on this paper, it can be estimated that to obtain lessthan 50 arcsec full width at half maximum (FWHM) for a triple-crystalx-ray rocking curve of the (0002) reflection (for a c-face wafer), theareal planar defect density is preferably below 100/cm².

The yield from a boule (the number of wafers that can be sliced from theboule that meet the size and defect specification) may be increased byreducing the areal density of planar defects in the boule and byincreasing the length of the boule. Preferably, a technologically usefulboule yields at least 5 wafers that meet the size and defectspecifications.

1. Preparation of the Seed Crystal

In the implementations discussed below, a high quality AlN seed crystalis prepared. The AlN seed crystal is preferably cut from asingle-crystal boule grown as described herein (i.e., a portion or allof a resultant boule is used to form seed plates for subsequent crystalgrowth). Typically the seed crystals are cut as round plates of about 2inches (50-60 mm) in diameter and having a thickness ranging from 0.2 upto 5.0 mm. However, smaller area seeds may also be prepared to be ableto select seeds formed from very high quality regions of a boule ofnonuniform quality or because a different crystal orientation isdesired. These smaller diameter seeds may be mined from AlN crystalboules grown as described herein. Seed plates or smaller area seedcrystals may also be prepared by slicing AlN boules fabricated by othertechniques, such as the technique described in the '660 applicationwhere a high quality encased AlN seed crystal, formed by selfnucleation, is used to seed the AlN crystal growth and the crystalgrowth crucible is arranged so as to expand the diameter of theresultant AlN boule up to 2 inches in diameter, as shown in FIG. 7 ofthat application. In all cases, it is important that high quality,nearly defect free seeds be selected, because defects in the seedcrystal(s) may be duplicated in the AlN boule to be produced. Inparticular, the areal density of planar defects in the seed crystals ispreferably below 100 cm⁻² and, even more preferably, below 1 cm⁻². Ifmultiple small area seeds are to be used simultaneously, the orientationof each seed is preferably carefully controlled so that the seeds can bematched when they are mounted on the seed holder.

The orientation of the seed crystal plate (or of the smaller seedcrystals) is typically with the c-axis parallel to the surface normal ofthe plate (a so-called c-axis seed plate), but other orientations andsizes are suitable as well. The surface of the AlN seed crystal thatwill face the seed holder assembly (the seed back side) is preferablysmooth and flat with a total thickness variance (TTV) of less than 5 μmand preferably less than 1 μm so that gaps between the seed crystal andthe seed holder assembly are reduced. A “smooth surface,” as usedherein, is a surface that has no scratches visible when viewed with anoptical microscope under 200× magnification and that the root meansquare (RMS) roughness measured with an atomic force microscope (AFM) isless than 1 nm in a 10×10 μm square area. Optical measurement techniquesare effective for measuring the TTV.

The top surface of the AlN seed crystal (which will serve as thenucleation site of the AlN crystal boule) is preferably smooth. Inaddition, any crystal damage in the top surface of the AlN seed crystalthat may have resulted from cutting or polishing the seed crystal ispreferably removed prior to attaching the seed crystal to the seedholder. This subsurface damage (SSD) layer may be removed in accordancewith methods described in U.S. Ser. No. 11/363,816 (referred tohereinafter as the “'816 application”) and Ser. No. 11/448,595 (referredto hereinafter as the “'595 application”), both of which areincorporated herein by reference in their entireties. An exemplarymethod includes performing a CMP step by applying an abrasive suspensionin a solution consisting essentially of a hydroxide. Another exemplarymethod is a CMP process that includes polishing a substrate using aslurry including an abrasive suspension in a solution capable ofmodifying the surface material of the substrate and creating a finishedsurface suitable for epitaxial growth. The active solution chemicallymodifies the surface of the substrate, forming a compound softer thanthe underlying substrate material. The abrasive is selected to be harderthan the newly created compound, but softer than the substrate material,so that it polishes away the newly formed layer, while leaving thenative substrate surface pristine and highly polished.

The specific recipe for SSD removal depends on the seed orientation.Removal of the SSD layer is important as it preferentially thermallyetches, leaving void and defect spaces as well as irregular topographyat the interface between the seed crystal and the resulting AlN boulethat may compromise crystal growth and may result in planar defects. Inparticular, improvements in polishing of the seed crystal may improvethe quality of the boule growth by reducing defects during thermalcycling. A suitable seed will have planar and/or extended voids of lessthan 1 per square centimeter intersecting either surface of the seed,less than one scratch of 10 nm depth within a 10×10 μm square AFM scanand less than one crack per cm².

Other defects that are preferably avoided include pits, grain boundaries(including polarity inversions) and cracks. In addition, surfacecontamination due to, for instance, polishing, handling, and oxidation,is undesirable. Void formation from the inclusion of scratched materialis a risk. Areas with SSD are more likely to thermally etch during theseed mounting heating cycle. Thermal etching of the AlN seed crystal orbacking material may create a void space. In addition, SSD representsdamaged crystal lattice within the seed crystal. Defective crystallattice within the seed crystal is generally replicated within the grownboule and may lead to the creation lower quality wafers that are cutfrom that boule. Thermal etching of the seed crystal may be mitigated byusing a lower mounting temperature (lower mounting temperature mayreduce thermal etching) or by gas species/pressure choices (highpressure N₂/argon/xenon, etc. may suppress thermal etching) but mayleave SSD that will be replicated in the seeded growth.

Voids present in the seed material may create voids in the grown boule.Voids intersecting the back surface of the seed may lead to seedmounting difficulties. Voids intersecting either the seed holder orgrowth interface surface of the seed may present contamination issues(trapped material). Therefore, seeds for seeded growth desirably areeither cut from boules that have been grown by these void-free methodsor cut from AlN boules generated by self-nucleation techniques describedin the '660 application.

In particular, as discussed in the '660 application, two conditions maybe considered to employ self-nucleation in the preparation of AlNboules. First, there is a nucleation barrier to the growth of AlN ontungsten. That is, the vapor above a tungsten crucible tends to besupersaturated unless AlN nuclei are available for growth. To takeadvantage of this, a seeded region may take up some part of the fulldiameter seed mounting plate that is surrounded by an unseeded, bareregion. Since adsorption of aluminum and nitrogen from the vapor ontothe seed is favored over deposition onto the bare crucible wall, theseed is favored to expand laterally in favor of creating new self-seededcritical nuclei next to the seed. Under properly controlled conditionsthis process can be used to increase the seeded area per growth cycle.Secondly, the process of crystal growth requires heat extraction whichis controlled by arrangements of insulators/heaters in the system.Properly arranging insulation so that the seed is the coolest part ofthe upper crucible and cooler than the source during growth is importantto the process. Further tailoring this insulation when using a smallseed to be expanded during the growth aids in expansion of the seed bymaking the seed cooler than the unseeded lateral region. This thermalarrangement makes self-seeded nucleations neighboring the seed lessfavored by limiting heat extraction. As the crystal grows at hightemperature and with sufficient source material, given sufficient timeto reach an equilibrium point during the growth run the interface of thecrystal will follow the isotherms of the system (insulation/heaters,etc.). The proper interface shape to favor seed expansion is slightlyconvex in the growth direction; the curvature of the gradient aidsexpansion.

Residual SSD may be identified and other defects such as threadingdislocations (TDD) may be revealed with a defect etch using a KOHvapor/solution or with a KOH-enhanced CMP, as described in Bondokov etal. in “Fabrication and Characterization of 2-inch Diameter AlNSingle-Crystal Wafers Cut From Bulk Crystals” [Mater. Res. Soc. Symp.Proc. Vol. 955 (Materials Research Society, Pittsburgh, 2007) p.0955-103-08]. The density of the pits measured in these defect etches isreferred to as etch pit density (EPD). For seeded growth, it isgenerally desirable to start with seeds that have less than 10⁴ EDP. Itis possible to improve grown boule over seed quality, but it ispreferable to start with high quality seeds. It is also important toavoid cracking the seed.

2. Detailed Example of a Seed Crystal Preparation

The procedure used to prepare the seed crystal surface depends on itscrystallographic orientation, as described in the '816 application and'595 application. Briefly, as described in those applications,crystallographic orientation affects mechanical preparation of asubstrate surface prior to CMP processing; substantial differences existfor optimal substrate preparation. For example, in the case of an AlNsubstrate, the Al-terminated c-face is not reactive with water, but theN-terminated c-face is reactive with water, along with non-polar faces.During wet lapping and polishing, the Al-polarity face tends to chipunder the same conditions that are well-suited to mechanically polishthe non-Al-polarity faces or Al-polarity faces where the c-axis isoriented 20 degrees or more away from the surface normal of thesubstrate.

Here, we describe an exemplary process for preparing a c-axis seed platewhere the nitrogen-polarity face (N-face) will be attached to the seedholder assembly and the aluminum-polarity face (Al-face) will be used tonucleate the AlN boule. After an appropriately oriented seed plate iscut from an AlN boule using a diamond wire saw (the seed plate is cutsuch that the c-axis is within 5° of the surface normal), the surfacesare ground flat and then diamond slurries (with progressively decreasingdiamond size) are used successively to further mechanically polish bothsurfaces of the seed plate. More specifically, the N-face of theas-sliced AlN wafers undergoes grinding (with 600 diamond grit), lapping(6 μm diamond slurry), and fine mechanical polishing with 1 μm diamondslurry. Then, the wafer is flipped over and the Al-face undergoesgrinding (with 600 and 1800 diamond grit), lapping (6 μm and 3 μmdiamond slurries), and fine mechanical polishing with 1 μm diamondslurry followed by the CMP, as described in the '816 application where ahigh pH silica suspension in a KOH solution is used to leave anAl-polarity, c-face surface that is free of SSD.

These mechanical polishing steps may be followed by a CMP step on theN-face of the seed crystal (which is the back surface that will bemounted facing the seed holder assembly in this example). A suitableslurry is a 1 μm Al₂O₃ slurry with active chemical solution (the slurryis made of 100 grams of 1 μm Al₂O₃ grit per 1 liter of solution composedof 0.5M KOH in distilled water (1 liter) with an additional 50 mL ofethylene glycol). The slurry is used on a soft composite iron polishingdeck such as the AXOS from Lapmaster, Inc.), leaving the surface highlyreflective to the eye and free of defects such as scratches or pits oropen cracks. The grit choice and active chemical reaction between theAlN and the strong base (KOH) are important for producing a surface withlow defect densities. A preferred surface has less than 1 scratch deeperthan 10 nm per 10 square μm scan with an AFM and the RMS roughnessmeasured with an AFM is less than 1 nm in a 10×10 μm area. In addition,the back side of the seed crystal surface preferably has a TTV of lessthan 5 μm and more preferably less than 1 μm. This is important becausesurface topography, even at a microscopic level, may result in planardefects forming in the seed crystal; these defects may propagate intothe crystal boule during subsequent growth. The flatness of the polishedsurfaces is checked using a suitable optical flat and monochromaticlight source (sodium lamp at 590 nm).

The Al-face is then subjected to a final CMP step after the 1 μm diamondpolishing step using a silica suspension from Cabot Industries (Cabot43). Additional techniques for preparing the surface of the seedcrystals are described in the '816 application and the '595 application.For example, as noted above, the CMP process may include polishing asubstrate using a slurry including an abrasive suspension in a solutioncapable of modifying the surface material of the substrate and creatinga finished surface suitable for epitaxial growth. The active solutionmay modify the surface of the substrate, forming a compound that issofter than the underlying substrate material. The abrasive may beselected to be harder than the newly created compound, but softer thanthe substrate material, so that it polishes away the newly formed layer,while leaving the native substrate surface pristine and highly polished.In some CMP processes, the slurry may include an abrasive suspension ina solution consisting essentially of a hydroxide.

The seed crystal is now ready for mounting on one of the seed mountingassemblies described below and is preferably carefully stored in anitrogen atmosphere glove box to avoid any contamination prior togrowth.

3. Seed Holder Plates

Different structures have been developed for the seed holder plate. Thepreferred approach depends on the particular circumstances used forcrystal growth.

3.1. Textured AlN Deposited on a Backing Plate

Referring to FIG. 8B, in an embodiment, a seed holder 800 may include arelatively thick, highly textured AlN layer, i.e., foundation 810,deposited on a metal backing plate 820, e.g., W foil. The holder 800 issized and shaped to receive an AlN seed therein. Preparation of apreferred embodiment may include one or more of the following threefeatures:

a.) The use of a seed holder including an AlN foundation bonded to anappropriate backing plate (in a preferred embodiment, this backing plateis W foil);

b.) Appropriately conditioning the backing plate so that it is nearlyimpervious to Al diffusion through the plate; and/or

c.) Using Al foil to form an adhesive 140 to bond the seed to the AlNceramic or seed plate by heating the seed plate/Al foil/AlN seed crystalto high temperature rapidly enough so that the Al first melts anduniformly wets the AlN with a very thin layer of Al before convertinginto AlN.

In an embodiment, the W foil has a thickness of 20 mils to 5 mils (510to 130 μm). A thinner W foil may be desirable to reduce the stress thatthe seed plate will apply to the seed crystal and the resulting bouledue to the thermal expansion mismatch between the AlN and seed mountingW plate. The thickness of foil used for the mounting plate may be chosensuch that the specific vendor/lot of W-foil provides a relativelyimpervious barrier to aluminum and/or nitrogen. This W-backing orbarrier layer is preferably made from high density material (fortungsten>98% theoretical density) and may be made of multiple layers ofgrains allowing grain swelling to close fast diffusion paths betweengrain boundaries. The latter approach has also been described in U.S.patent application Ser. No. 11/728,027 (referred to hereinafter as the“'027 application”), incorporated herein by reference in its entirety.As discussed therein, machining of powder metallurgy bars includingtungsten grains having substantially no columnar grain structure is anexemplary method of forming multilayered and/or three-dimensionalnominally random tungsten grain structures that can help preventpermeation of aluminum through the tungsten material. In addition, thisW backing plate may be made from single crystal tungsten that may nothave any grain boundary diffusion.

The W foil is preferably cleaned and conditioned with aluminum prior tocrystal growth. The foil may be further conditioned by applyingadditives such as Pt, V, Pd, Mo, Re, Hf, or Ta. Thicker layers oftungsten may be used to limit Al diffusion through the backing plate butthey will suffer from increased thermal expansion mismatch between thematerials leading to higher cracking densities in the grown AlNcrystals.

The polycrystalline W foil is preferably made of layers of grains. Thesestacked and compressed pure W-grains contain path ways between thegrains (where the grains meet neighboring grains) that allow diffusionpaths between the grains. Loss of aluminum is primarily through thesegrain boundaries and leads to voids (planar or extended) in the AlN. Intime, as these W grains absorb Al atoms through diffusion into the Wgrains, the W grains will swell as much as 5%, as Al is a substitutionalimpurity in W and has approximately a 5% solubility. As detailed in the'027 application, these swollen grains will decrease the grain boundarydiffusion rate. The Al-conditioning may be achieved at growthtemperature by processing similarly to the described AlN-foundationprocess. Rather than using Al to condition the W foil, other materialssuch as Pt, V, Pd, Mo, Re, Hf, or Ta may be used to decrease the amountof Al lost through grain boundaries by swelling, filling or decreasingthe grain boundary density in the W backing plate.

In the cases of Pt, V, or Pd, the elements may be applied (painted,sputtered, plated or added as foils) to the W foil and run through aheating cycle, preferably above the melting point of the added materialbut below the melting point of the tungsten, to allow the added elementto melt, leading to reaction with the W grains. This tends to cause theW grains to swell and to decrease both the time and Al required tofurther swell the grains and reduce Al losses through grain boundarydiffusion.

In the cases of Mo and Re, the elements may be mixed with the W to forman alloy. These alloys have a lower eutectic point with the Al presentunder growth conditions. This means that backing material composed ofthese alloys may not be suitable at as high a growth temperature as puretungsten. The lower eutectic point means that exaggerated grain growthtends to be faster than pure W with the same Al exposure conditions.While care must be taken to ensure that there are enough layers ofgrains in these alloy foils, the surface layers of grains will quicklyswell on exposure to Al vapor, which will prevent further Al diffusionalong the their grain boundaries. An additional advantage of Mo and Realloys with tungsten is that these alloys may have a smaller thermalexpansion mismatch with AlN, which will improve the cracking yield(i.e., fewer boules will be cracked).

In the cases of Hf and Ta, the applied layers on the W-foil may bereacted to form additional film or barrier layers on the W foil whichwill help to fill the grain boundaries in the W foil. The Hf or Ta canbe applied to the W-foil surface by adding powder, foil, sputtering orplating. The pure element spread over the polycrystalline W foil canthen be reacted with nitrogen or carbon to form HfN, HfC, TaC or TaNwhich will aid in sealing grain boundaries and will reduce the grainboundary diffusion rate through the W foil. These nitride or carbidecompounds could be applied directly as well provided they could beapplied in continuous layers forming a minimum of additional pathways orgrain boundaries through the layer.

Referring to FIG. 9, a single-crystal seed 100 is attached to a seedholder 800 using a weight 900. The single-crystal seed 100 attached tothe seed holder 800 by, e.g., adhesive 140. The important elements ofthis approach are: (i) that the AlN foundation, if properly formed,provides a nearly perfect thermal expansion match to the growing AlNboule as well as an excellent chemical match; (ii) the backing plate,when properly conditioned, provides a nearly impervious barrier to Aldiffusion; and (iii) the rapid thermal processing of the Al foil alongwith excellent polishing of the AlN seed and the AlN foundation,provides a tight and dense bond between the foundation and the seed thatwill help prevent planar defects from forming

4. Preferred Implementations

In a preferred embodiment, a polycrystalline AlN foundation is producedby the sublimation-recondensation technique described in the '660application, in which a relatively thick (3 to 5 mm) layer of AlNmaterial is deposited directly onto a metal foil or plate. The processincludes sublimation, wherein the source vapor is produced at least inpart when crystalline solids of AlN or other solids or liquidscontaining AlN, Al, or N sublime preferentially. The source vaporrecondenses on a growing seed crystal. It may be desirable to have thethickness of the AlN deposit be more than 10 times the thickness of themetal plate so that the relative stiffness of the AlN layersubstantially exceeds that of the metal plate. In this manner, themajority of the strain from any thermal expansion mismatch between themetal plate and the AlN foundation plus seed crystal (plus crystal bouleafter growth) may be taken up by the metal plate. It may be desirable tonot have the thickness of the foundation layer be too large, because agreater thickness may limit the size of the eventual crystal boule to begrown. For this reason, the thickness is preferably limited to less than20 mm We have found that deposition of the AlN under typical growthconditions described in the '660 application can result in a highlytextured AlN film. In this context, a textured film means that almostall of the AlN grows in the form of grains having a c-axis (the [0001]direction using standard notation for hexagonal crystals) orientedparallel to the surface normal of the growing film. The diameter of thegrains in the plane perpendicular to the growth direction (i.e.,perpendicular to the [0001] crystallographic direction) is typically 0.1to ˜2 mm in size. An advantage of this highly textured film is itsbeneficial impact, derived from the fact that AlN has various thermalexpansion coefficients that depend on the crystallographic direction. Apolycrystalline film where the individual grains were randomly orientedmay crack as it is cycled from the growth temperature of approximately2200° C. to room temperature.

While the AlN is being deposited on the W foil, the surface of the Wfoil may become saturated with Al which, we have observed, will greatlyreduce further diffusion of Al through the foil. This phenomenon isdescribed in '027 application, where it is noted that the penetrationrate of aluminum along grain boundaries is reduced after the tungstengrains have swelled due to uptake of Al by bulk in-diffusion. It may bedesirable to form the polycrystalline W foil so that it containsmultiple layers of W grains. We have found that W foil that is 0.020 to0.005 inch thick (e.g., material supplied by Schwarzkopf, H C Starck, HCross) is satisfactory for this purpose. Other metal foils or plates arealso suitable; these include Hf, HfN, HfC, W—Re(<25%), W—Mo(<10%),pyrolitic-BN (also called CVD-BN), Ta, TaC, TaN, Ta₂N, carbon (vitreous,glassy, CVD, or POCO) and carbon coated with Ta/TaC, Hf/HfC and BN. Wehave also found it helpful (depending on the grain structure of thefoil) to precondition the W foil by exposing it to Al vapor and lettingthe surface layer saturate with Al prior to significant deposition ofthe AlN layer on top of the foil.

Following growth and cool down of the polycrystalline AlN layer on thebacking material (or as-grown foundation), the foundation may beinspected to determine the suitability of the as-grown foundation forfurther use in seed mounting. In some embodiments, suitable AlNfoundations exhibit no or low cracking (<1 crack per square cm), no orlow planar voiding (<1 planar intersecting the surface per square cm),and no or low areas of thin AlN deposition (sufficient grown thicknessto polishing to specification). Inclusion of cracks, voids or thinlayers behind the seed mount area may create void space behind the seedcrystal. This void space may migrate, as described previously, todeteriorate the seed crystal and grown AlN boule.

In the described configuration, the AlN foundation layer may act toreduce the forces from thermal expansion mismatch on the grown boule bymatching the thermal contraction of the grown AlN boule. The holderplate (backing layer of W-foil) acts as the layer that is relativelyimpervious to aluminum and/or nitrogen barrier layer preventingmigration of the crystal material leading to void formation.

After the AlN layer is deposited as described above, it is preferablypolished to a smooth and flat surface. As mentioned above, a “smoothsurface” in this context means that there are no visible scratches in anoptical microscope (200× magnification) and that the root mean square(RMS) roughness measured with an atomic force microscope (AFM) is lessthan 1 nm in a 10×10 μm area. This is important as surface topography,even at a microscopic level, may result in planar defects forming in theseed crystal; these defects may propagate into the crystal boule duringsubsequent growth. The flatness of the polished foundation surface maybe checked using a suitable optical flat and monochromatic light source(sodium lamp at 590 nm is typical). The foundation surface is preferablyflat across the seed area to better than 5 μm and preferably better than1 μm. The as-grown AlN foundation layer on the W seed backing foil ispolished in the manner of a fine mechanical preparation of asingle-crystal AlN substrate, e.g., as described in '816 application. Inan exemplary CMP process, substrate may be polished with a slurryincluding an abrasive suspension in a solution, such that the slurry iscapable of etching the substrate surface and creating a finished surfacesuitable for epitaxial growth. A silica suspension in a hydroxidesolution may be used, e.g., the KOH-based CMP slurry known in the art asSS25 (Semi-Sperse 25) available from Cabot Microelectronics or the Sytonslurry available from Monsanto. The W foil backing side of the AlN/Wfoundation (as grown) may be mounted to a polishing fixture using asuitable mounting adhesive (e.g., Veltech's Valtron—AD4010-A/AD4015-B—50CC thermal epoxy). The rough shape of the composite may be leveled bypolishing the AlN layer using a rough mechanical step. A suitableapproach is to use a 15 μm diamond slurry on a steel polishing deck(e.g., a Lapmaster 12″ or an Engis LM15 with a regular steel deck). Thisrough mechanical step may be followed by a fine mechanical process with1 μm Al₂O₃ slurry in a KOH solution (the slurry is made of 100 grams of1 μm Al₂O₃ grit per 1 liter of solution which is composed of 0.5M KOH indistilled water (1 liter) with an additional 50 mL of ethylene glycol).The composite is polished with this slurry on a soft composite ironpolishing deck such as the AXOS from Lapmaster, Inc.), leaving thesurface highly reflective to the eye and free of defects such asscratches or pits or open cracks. The grit choice and active chemicalreaction between the AlN and the strong base (KOH) is important toproduce a surface with low defects. The preferred surface has less than1 scratch deeper than 10 nm per square 10 μm scan (AFM) and TTV of lessthan 5 μm across the seeded area. In addition to providing this flat,scratch-free surface, the chemical reactivity of the solution and lowhardness (with respect to AlN) of the grit and deck material providessufficiently low SSD to avoid thermal etching the AlN foundation.

Following a suitable polishing process, the foundation is chemicallycleaned of polishing residues prior to the described seed-mountingstages involving the foil and seed.

4.1. AlN-to-AlN Bonding Using Al Foil Nitridation

The AlN seed is now bonded to the AlN foundation using Al foilnitridation. The Al foil is placed between the seed and the foundation,and is heated up to temperatures sufficient to nitride the whole Al foiland thus produce a thin AlN bonding film between the AlN seed and AlNfoundation. In other words, the Al foil is interposed between the seedand the foundation, and melted to uniformly wet the foundation with alayer of Al. The Al foil nitridation has the advantages of cleanlinessand producing a microscopically conformal coverage of the seed backside,resulting in low planar-defect densities. The density and chemicalstability of any backing material used to protect the seeds areimportant. If the backing material is not chemically stable (e.g.,against Al vapor), then the resulting reaction between the Al vapor andthe backing material may result in decomposition and thus voiding. Ifthe backing material is not dense enough, the Al vapor can sublimethrough it, leaving behind extended voids and/or planar defects. If thebacking material has a high vapor pressure at AlN crystal growthconditions, then it will migrate allowing void formation and willpossibly become a boule contaminant. A schematic diagram of thisstructure is shown in FIG. 8B.

AlN seeds are known to form oxides and hydroxides during exposure toair, moisture and during chemical cleaning (hydrous and anhydrouschemicals contain enough water to react given AlN properties). As such,the prepared and cleaned seed surfaces may have some reproducible layerof oxides or hydroxides present during seed mounting. One advantage ofusing a liquid flux (the aluminum metal is melted and remains a liquidbefore forming a nitride and becoming a solid during the describedprocess) is that the liquid will dissolve the seed surface oxide priorto reaction and convert the oxide into a more stable form and/ordistribution. A layer of oxide and/or hydride on the seed surface mayhave a high vapor pressure under growth conditions and may lead to voidformation. The more chemically reactive side of the MN c-axis wafer (theN-face) will have hydroxide formation that may be >10 nm in thickness.

The starting materials for an exemplary process are a polished AlNfoundation seed holder, polished AlN seed crystal, and Al foil (10 milthick from Alfa Aesar). First the materials are cleaned to producereproducible and clean surfaces. The AlN foundation seed holder,prepared as described above, is treated as follows:

1. HCl:H₂O [1:1] boil to remove polishing residues (20 min)

2. Distilled water rinse

3. Room temperature HF (49% solution) dip (15 min)

4. Anhydrous methanol rinse 3 times

5. Store under anhydrous methanol while assembling seed mount.

6. Dry carefully to avoid solvent stains upon removal from the anhydrousmethanol.

The AlN seed crystal (after preparation as described above) is treatedas follows:

1. HCl boil to remove remaining epoxy residues from boule processing (20min)

2. Room temperature HF (49% solution) soak to remove SiO₂ and polishingresidues (15 min) and surface oxide/hydroxide layers.

3. Anhydrous methanol rinse 3 times

4. Store under anhydrous methanol while assembling seed mount

5. Dry carefully to avoid solvent stains upon removal from the anhydrousmethanol.

The Al foil is treated as follows (Al-foil: 10 μm thick, 99.9% purityfoil provided by Alfa Aesar is preferred embodiment):

1. Cut to square sufficient to cover the seed area

2. Drip (1 min) in HF:HNO₃ solution (RT) for 1 min—removes oil andoxides

3. Anhydrous methanol rinse 3 times

4. Store under methanol while assembling seed mount

5. Dry carefully to avoid solvent stains upon removal from the anhydrousmethanol.

With cleaned components:

1. Remove foundation from anhydrous methanol

2. Remove Al-foil from anhydrous methanol

3. Place foil dull side down and smooth side up onto the foundation

4. Smooth any air bubbles from behind the foil so that the thin/softfoil is void free on the foundation.

5. Remove the seed from the anhydrous methanol

6. Place seed (polarity determined) onto the foil

7. Trim excess foil from around the seed with a clean razor blade.

The seed, foil, and foundation are stacked into the furnace (invertedfrom the orientation shown in FIG. 8B to obtain the orientation shown inFIG. 9, so that gravity holds the seed and foil down on the foundation).Clean W weights 900 are then stacked on the seed to ensure that, duringthe melt phase, the seed is pressed toward the mount surface to reducegaps. In an exemplary embodiment, about 0.6 kg of W mass per 2″ wafer.The W weights are cleaned prior to use by heating in a furnace in areducing atmosphere (typically forming gas is used with 3% hydrogen) toa temperature higher than the seed-mounting temperature for severalhours and polished flat by mechanical polishing processes similar to thefoundation and seed process/equipment.

Once the stack of weights, seed, foil, and foundation are positioned inthe furnace, the station is evacuated to base pressures<10⁴ mbar,preferably <10⁻⁶ mbar and refilled with clean gas (filtered UHP gradeforming gas (3% H₂ and 97% N₂).—lower than 1 ppm impurity of moisture,oxygen, hydrocarbons). Preferably, a station is used that is capable ofhigh purity gas flow through the reaction zone where the seed ismounted. The flow gas tends to act as a curtain of clean gas, keepingchamber contamination away from the seed mount area. Contamination ofthe seed mount process may lead to the formation of oxides, carbides,materials other than pure AlN, and pure seed backing material mayintroduce unstable species that may migrate during crystal growth,leaving space that may allow void formation. Seed mount or bondingcontamination (oxide formation) may lead to lower thermal conductivityregions behind/around the seed. Preserving consistent and high qualitythermal contact around the seed and to the seed backing is important formaintaining good seeded growth. Oxides and other impurities tend to havea higher vapor species during crystal growth leading tomigration/sublimation of the contaminant causing void spaces.

As mentioned above, gas flow is one way to improve the purity of theseed mount. A second way is to introduce a getter, with current bestpractice using both gas flow and getter materials. The preferredgettering materials are yttrium metal and hafnium metal. These act togetter the local atmosphere of contamination around the seed duringmounting. The yttrium metal melts at 1522° C. (during ramp up of theAl-foil seed mounting process) and spreads to getter a wide surfacearea. Using a thin foil of the material tends to be most effective(e.g., Alfa Aesar, 0.1 mm thick, 99.999% purity Y-foil). Furthermore,yttrium oxide is stable under typical AlN growth conditions, meaningthat it will provide only a low vapor pressure of oxide contaminationback into the crystal growth environment if this getter from the seedmount remains during the growth. Hafnium-metal getter will not melt(melting point>2200° C.) under the described seed mount conditions buttends to surface react with both the oxide and the nitrogen. Therefore,the powder form of hafnium is preferred for this application (e.g., AlfaAesar, −325 mesh, 99.9% metal basis purity). Each of these getters canbe cleaned prior to use or purchased in sufficient purity to be used forthe described application (99.9% or purer is current practice).

In each case, getter materials are placed around the periphery of theseed mounting area at the edges of the seed holder to avoid impuritiesfrom entering the seed bonding reaction zone.

In the case of hafnium powder, the hafnium will readily nitride underthe described process. The HfN layer created in at the powder level orat higher temperatures (when the Hf melts and spreads at 2205° C.)forming a HfN layer. It has been observed that the HfN layer acts toprevent W-components from sticking together, even following long heatingcycles with Al-vapor present. This property allows surfaces to beprepared that will not stick together, despite being well polished andvery clean in the hot/reducing atmosphere.

After these steps, the seed mounting setup is ready for the heatingcycle. In a preferred embodiment, the seed-mount stack is rapidly heated(<5 minutes) to approximately 1600° C. and ramped in 30 minutes to 1650°C. The purpose of this is to quickly melt the Al foil and to allow theAl liquid to readily flow with low surface tension, allowing the Al meltto readily wet the AlN seed crystal and the AlN foundation uniformly,i.e., melting the Al foil to uniformly wet the foundation with a layerof Al. A high density AlN between the original seed crystal and the AlNfoundation is formed. Allowing the heat-up cycle to remain at lowtemperatures (below about 1100° C.) for too long may permit the liquidAl to bead up and form a porous AlN ceramic when the Al starts tonitride, thereby creating void spaces behind the seed. Once at 1650° C.,the temperature is held for >1 hour to allow the Al-melt to fullynitride, forming a high-density AlN ceramic that is bonded to the seedand to the AlN foundation. Following the >1 hour soak at 1650° C., thestation is ramped to room temperature in an additional 2 hours.

Following this heat cycle/nitride mounting, remaining getter materialsand seed mounting weight 900 are removed from the assembled seed-seedholder. The seed and seed holder assembly is now ready to be inverted asshown in FIG. 8B and assembled for the crystal growth cycle. The crystalgrowth crucible is assembled as shown in FIG. 10. In particular, AlNseed 100 and seed holder assembly (including adhesive 140, foundation810, and backing plate 820) are assembled as shown in a crystal growthcrucible 1100. The AlN seeded bulk crystal growth may be carried out ina tungsten crucible 1100 using a high-purity AlN source 1120. The AlNseed 100 is mounted onto the seed holder assembly as described above.

Single-crystal aluminum nitride is formed by depositing aluminum andnitrogen onto the AlN seed 100 under conditions suitable for growingsingle-crystal AlN originating at the seed. For example, growth may beinitiated by heating the crucible with the seed mount and sourcematerial to a maximum temperature of approximately 2300° C. and with agradient of less than 50° C./cm as measured radially and a verticalgradient greater than 1° C./cm but less than 50° C./cm. During theinitial ramp-up to the growth temperature, it may be desirable toposition the seed crystal and the source material such that they are atapproximately the same temperature (the seed equilibrium position) sothat any impurities on the surface of the seed crystal are evaporatedaway prior to growth. Once the growth temperature is achieved, it may bedesirable to either move the crucible assembly so that the seed istemporarily hotter than the source material, or to temporarily reducethe nitrogen partial pressure prior to initiating growth on the seedcrystal in order to evaporate part of the surface of the seed crystal.The partial pressure of nitrogen in the furnace may be reduced either byreducing the total pressure of gas in the furnace or by adding an inertgas, such as Ar, to the furnace while keeping the total pressure in thefurnace constant.

The bulk single crystal of AlN formed by this method may have a diametergreater than 20 mm, a thickness greater than 0.1 mm, and an areal defectdensity≦100 cm⁻². The method may enable the formation of bulk singlecrystal AlN in the form of a boule having a diameter greater than 20 mm,a thickness greater than 5 mm, and an areal density of threadingdislocations≦10⁶ cm⁻²—or even ≦10⁴ cm⁻²—in each cross section of thebulk single crystal disposed in a plane perpendicular to a growthdirection of the crystal. A boule may include a bulk single crystal ofAlN having a sufficient thickness to enable the formation of at leastfive wafers therefrom, each wafer having a thickness of at least 0.1 mm,a diameter of at least 20 mm, and a threading dislocation density≦10⁶cm⁻², preferably ≦10⁴ cm⁻².

A boule formed by methods described herein may be a substantiallycylindrical bulk single crystal of AlN having a diameter of at least 20mm and having a sufficient thickness to enable the formation of at leastfive wafers therefrom, each wafer having a thickness of at least 0.1 mm,a diameter of at least 20 mm, and a triple-crystal X-ray rocking curveof less than 50 arcsec FWHM for a (0002) reflection, with each waferhaving substantially the same diameter as each of the other wafers.

4.2 Multiple Seed Mounting

It may be desirable to mount several seeds on the AlN ceramicsimultaneously. For instance, it may be difficult to obtain seedcrystals large enough, with sufficiently high quality, to cover theentire area of the AlN ceramic. In this case, it may be desirable to usemultiple seeds that may be mounted on the AlN ceramic simultaneously.This may be accomplished by preparing seed crystals as described above,all with the same orientation. The seed crystals may then be mounted onthe AlN ceramic on the metal backing plate as described above (or otherseed holder assemblies as described below) with careful attention toaligning their azimuthal axis. In the case of smaller seeds, it ispossible to expand a seed within a growth run using thermal gradients.The laterally expanded seed crystal generally avoids the seed mountingsource of planar voids but may still require a low porosity seed backingbarrier to avoid through voiding formation of planar defects within thegrown boule. In addition, it may be possible to arrange a patch work ormosaic of small seeds accurately enough so that the resulting largediameter boule is grown with suitable orientation between the smallerseeded regions to produce a congruent 2″ wafer. For c-axis AlN seededgrowth, the alignment of the seeds is preferably accomplished bypreparing the seed crystals with m-plane cleaved edges. The AlN cleaveson the m-plane to produce very straight edges perpendicular to thec-axis. Thus, the seeds may be well oriented with respect to each otherby aligning the flat m-plane cleaves against the neighboring seedsections. From a small seed mosaic approach, a fraction of the 2″ waferusable area may easily be produced, but it may also be possible to seedthe entire 2″ area by this method. A particularly important example ofusing more than one seed crystal is when a 2″ seed crystal is crackedand this seed crystal is mounted with the two halves aligned preciselyfor boule growth. By using this m-plane cleave face alignment approachto c-axis seeded growth it is possible to achieve <0.5 deg m-plane andc-axis crystallographic alignment. Because of the difficulty inobtaining seed crystals that are all exactly aligned and the difficultyin avoiding some error in the aligning of the azimuthal axes, thisapproach typically produces a higher defect density than a single seedcrystal. However, this approach may be used to obtain larger AlN crystalboules with smaller seed sizes.

4.3 Additional Approaches that May be Used to Supplement the PreferredImplementations

4.3.1 Protection of the AlN Seed Using Relatively Impervious Films

The back of the AlN seed may be protected by depositing ahigh-temperature, relatively impervious material like W. This barrierlayer can be deposited by sputtering, CVD, ion deposition or plating(for conductive substrates). Plating may be used to initiate or thickenthe deposited layer of seed back sealant once initial deposition hasbeen performed. For instance, the back of the AlN seed can be protectedusing W film sputtered onto the back of the AlN seed and then mounted tothe seed holder using any of the techniques described above. The back ofthe AlN seed may also be protected by attaching it (with an adhesivesuch AlN which is formed by nitriding a thin foil of Al as was describedabove) to W foil. The W foil may be single crystal to reduce Aldiffusion. The density of the planar defects is then reducedsignificantly. Other materials expected to possess suitable propertiesto be used as relatively impervious barriers include: Hf, HfN, HfC,W—Re(<25%), W—Mo(<10%), pyrolitic-BN (also called CVD-BN), Ta, TaC, TaN,Ta₂N, Carbon (vitreous, glassy, CVD, POCO) and carbon coated withTa/TaC, Hf/HfC and BN. The key attributes of a suitable material to bedeposited on the back surface include:

a. Temperature stability (>2100° C.)

b. Chemically stable in growth environment (Al-vapor, N₂, H₂)—vaporpressures<1 mbar at temperatures>2100° C. in N₂, N₂—H₂(<10%), Ar, around1 atm pressure.

c. Low diffusivity of Al through the backing material by beingphysically impervious to gas flow (generally this means that thematerial is dense without voids) and having a small diffusion constantfor Al. Since diffusion along grain boundaries is generally much higherthan diffusion through grain boundaries, it may be desirable to have thegrains swell so as to become more dense as Al diffuses into the material(“self-sealing” grain swelling as described in the '027 application.)

The material may, for example, be exposed to Al vapor prior to use as aseed holder plate to limit Al diffusivity through grain swelling in theplate. At typical growth temperatures, the vapor pressure in the growthatmosphere is about 0.1 bar Al-vapor and the equilibrium (atom-wt-%) Allevel in W has been measured to be ˜5%, so the preferred backing willhave no voids, will not evaporate or migrate during the run, and willhave its surface pre-saturated with the equilibrium Al-content for thatmaterial at the anticipated growth temperature.

4.3.2. Growth of Bulk AlN Single-Crystals Along Off-Axis Directions

The AlN bulk crystal may be grown parallel to directions at least 15±5°off-axis. The off-axis growth include crystal growth with interfaceparallel to non-polar {1 1 00} and semi-polar planes {1 0 1 1}, {1 1 02}, and (1 0 1 3). In the case of non-polar growth, the growth rate ofthe crystallographic planes differs from the growth rate of the sameplanes when crystal is grown on-axis or slightly off-axis. Therefore,even though the back surface of the seed may not be perfectly protected,planar-defect formation may be resolved into generation of otherdefects, e.g., stacking faults, twinning, etc., to reduce its impact.

4.3.3 Protection of the Back of the Backing Plate (Outer Sealing)

In addition to mounting the AlN seed onto the seed holder as describedabove, the outside of the seed holder (i.e., backing plate 820 in FIG.8B), that forms the crucible lid—i.e., the side outside the crucible—maybe protected to inhibit the transport of Al through the crucible lid.For purposes herein, high-temperature carbon-based adhesives, paints, orcoatings may be applied. Typically these materials are applied bybrushing or spraying and then thermally cycled to improve their densityand structure, but they may be sputtered or electro-plated as well. Forinstance, if a thin (<0.005 inch) W foil is used as a crucible lid andwith the AlN seed mounted on one side, then the other side of the W foilmay be protected in this fashion. The advantage of protecting the outerside of the foil is that a much wider range of high-temperaturematerials (coating, paints, etc.) may be used as the protective layer,since there is a lower risk of interaction between Al vapor and theprotective material. This approach also allows thinner metal lids to beused, which is advantageous in reducing stress on the crystal due tothermal-expansion mismatch between the lid material and AlN.

4.3.4 Seed Bond Curing in Multiple Gas Species Flow

As mentioned above, within at typical Al-foil seed mounting process, theliquid Al-foil cleans the seed surface of oxides and reacts to formAl₂O₃. To move to fewer voids and better quality growth, it may benecessary to more fully remove this seed oxide layer. Extending the timethat the Al-foil melt is allowed to react with this oxide layer is onemethod for doing this. The longer Al-melt phase may be achieved byreducing the amount of nitrogen available to react with the moltenAl-metal forming solid nitride. This can be performed under an argonatmosphere during heat up to suitable reaction temperature (1000 to1800° C. depending on desired removal rate/species) and holding forsufficient time to remove oxide and hydroxide layers from the seed.Subsequently, nitrogen may be added to the flow past the seed mountzone. The nitrogen may then react with the free Al-melt and form anitride seed adhesive.

During this molten Al phase, it is possible that the seed holder (whenmade of W alone) will be a diffusion membrane for the oxide species.This mechanism would allow the getter of the oxide from the seed to beachieved by the Al-metal, the metal to be cleaned by the W-layer andthen pure, high density AlN to be nitrided from the Al-melt forming ahigh quality seed adhesive.

4.3.5 Seed Bonding Directly to a Seed Plate without the AlN Layer

Rather than using a combination of an AlN ceramic layer and a backingplate, it may also be possible to bond the seed directly to anappropriate seed plate without the intermediary AlN ceramic layer. Thismay provide the advantage of eliminating the potential for defects inthe AlN ceramic layer to migrate into the growing AlN boule. However,the backing plate is carefully chosen so as to not introduce too muchstress onto the seed crystal and AlN boule due to thermal expansionmismatch between the seed plate and AlN. This can be accomplished eitherby using very thin plates that will easily deform in response to stressfrom the AlN crystal (yet still be relatively impervious to Al transportthrough the plate) or by using plates that relatively closely match thethermal expansion of AlN from room temperature up to the growthtemperature of ˜2200° C. Alternatively, the AlN seed crystal may bemounted on the backing plate, which may then be mounted on a texturedAlN ceramic. This last approach is attractive because the seed backingplate used may provide a relatively impervious barrier to Al diffusionand prevent defects from the AlN ceramic from diffusing into the growingcrystal. However, the AlN ceramic may provide the mechanical strength tohold the growing crystal boule.

Possible choices include:

i. W-foil

ii. W—Re foil

iii. W—Mo foil

iv. W-foil treated with Pt, V, Y, carbon

v. Single crystal-W backing

vi. HfC—liquid phase sintered

vii. TaC coated Ta

viii. TaC coated pBN

ix. TaC coated W-foil

x. HfN coated W-foil

xi. HfC (hafnium carbide)

xii. HfC coated W

xiii. BN coated graphite

Even though the W has a thermal expansion coefficient different fromthat of AlN, thin W-foil and thin single crystal-W may mechanicallydeform much more readily than an AlN boule of suitable thickness so asto reduce stresses on the crystal due to the thermal expansion mismatch.Alloys of W/Re and W/Mo may be selected such that the total thermalexpansion of the seed holder and AlN will be zero from growthtemperature down to room temperature. Combinations of these materials(all) and treatments with elements such as Pt, V, Y, carbon may be usedto change the grain growth behavior of the backing material to reducethe time dependent grain growth of the material upon exposure to Al andhigh temperature gradients.

A similar polishing preparation process to what was described above forthe AlN ceramic foundation is also suitable for direct foil mounting(without AlN foundation). To improve the surface finish further in thecases of metal backing it is generally desirable to follow the 1 μmAl₂O₃ deck step with a 1200-grit pad step that produces a minor finishon the softer metal materials while maintaining flatness and lowscratching.

The furnace operation for this seed mounting process is schematicallydescribed below. The adhesive layer is place on the prepared seed holderand the seed onto the adhesive layer. For use of the Al-foil based seedmounting adhesive, the seed holder from FIG. 6 is assembled whileinverted within a station capable of reaching at least 1650° C. Formaterials other than Al-foil, a separate heating cycle is describedlater, however, the same considerations apply to maintaining highquality seeded growth results.

A suitable mass is placed on top of the seed/adhesive/seed holderassembly. In an embodiment, one may use a polished (flat) tungsten rightcylinder that has been carefully out-gassed of contamination by repeatedheating cycles under forming gas flow. The block presses on the polished(flat) seed face with a pressure greater than 150 grams per centimetersquared area. In this case, this may be sufficient to hold a flat,stress relived seed closely against the seed holder. More pressure perarea will help to improve imperfect seed/seed-holder flatness bydeformation of the materials up to the point where the mass loading maycause seed/seed-holder fracture by exceeding the critical resolved sheerstress (CRSS) at room or higher temperatures.

Prior to seed assembly, the seed and seed holder are typically checkedfor suitable flatness using optical flatness measurement techniques suchas an optical flat and a monochromatic light source (435 nm sodiumlamp). Gaps between the mating surfaces are preferably less than 5 μm,preferably less, with part shapes being regular (avoid cupped or boxedpieces with deformation better than 5 μm preferred).

4.3.6 Other Possible Seed Mounting Adhesives

Instead of an AlN ceramic-based adhesive, it is possible to use anyother high-temperature adhesive, e.g., carbon-based adhesives or evenwater-based carbon paints such as Aquadag E, molybdenum-dag, (such asfrom Aremco Products, Inc.) molybdenum-powder or foil, molybdenumsputter or plated coatings, similar to each of the molybdenum formsincluding base elements aluminum, rhenium, vanadium, yttrium Otherglues, such as boron nitride-, zirconia-, yttrium oxide-, and aluminumoxide-based glues that have a variety of high temperaturestabilities/suitability at AlN growth conditions may also be used.

The carbon-based approaches have been successful for seeding SiC crystalgrowth. However, they have not proven successful for AlN crystal growthbecause Al vapor attacks the graphite forming aluminum carbide (Al₄C₃).

4.3.7 Using a Liquid or Break-Away Seed Mounting

As discussed above, one of the difficulties of growing bulk AlN fromseed crystals mounted on seed holders that are nearly impervious to Altransport is the strain caused by the thermal expansion mismatch betweenthe seed crystal and the seed holder plate. Stress from thermalexpansion mismatch can be avoided by using a liquid or nearly liquidfilm to hold the seed to the seed holder plate. Metal gallium (Ga) maybe substituted for one of the solid glues described above and will meltat 30° C. At high temperatures (>1,000° C.), the nitrides of Ga are notstable so the Ga will remain liquid between the AlN seed and the seedholder plate and thus will not be able to transmit any shear stress (dueto thermal expansion mismatch) to the growing AlN boule. However, theliquid Ga typically forms a nitride as the crystal is cooled to roomtemperature. This may be avoided by using a backing plate from which theGaN will break away as it cools or by replacing the nitrogen gas in thegrowth chamber with an inert gas (such as Ar) so that the Ga will not beexposed to enough nitrogen to form a solid nitride bond both the seedcrystal and the seed holder plate. Of course, this approach may notprovide any mechanical strength to hold the seed crystal, so it ispreferably used by mounting the seed crystal at the bottom of the growthcrucible.

The relatively high vapor pressure of the Ga may cause contamination ofthe growing AlN crystal boule. This may be overcome by using a eutecticof gold and germanium. The Au_(x)Ge_(1-x) has a eutectic at x=0.72 whichmelts at 361° C. Again, this material does not have any stable nitridesat the AlN growth temperature and, thus, will remain liquid. Inaddition, its vapor pressure will be approximately 30 times lower thanthat of Ga at the same temperature.

4.3.8 Seed Mounting without a Holder Plate

A large, low defect seed crystal may also be mounted by coating its backsurface with a nearly impervious coating and using the seed crystalitself to seal the crystal growth crucible. By making this coating thin,mechanical stresses from the thermal expansion mismatch between thecoating and the AlN seed crystal will be minimized. In the preferredembodiment of this approach, the seed crystal is first coated in DAG andthen baked at 150° C. to provide a carbon coating around the entire seed(alternative carbon coating schemes may also be used). The carbon coatedAlN seed crystal then has a thin layer of pyrolytic BN deposited on it(this layer is preferably approximately 100 μm thick). After thispreparation, the front surface of the AlN seed crystal is polished asdescribed above in the section on seed crystal preparation, so that thefront surface has substantially all of the BN and graphite removed, andis smooth and relatively defect-free as described in that section. Thisintegrated seed crystal and seed holder assembly will then be mounteddirectly as the lid for the AlN crystal growth crucible.

5. Additional Thermal Gradient Approaches

FIG. 11 depicts a crystal-growth apparatus 1100 suitable for the growthof single-crystal semiconductor materials (e.g., AlN, Al_(x)Ga_(1-x)N,B_(x)Al_(1-x)N, and/or B_(x)Ga_(y)Al_(1-x-y)N) in accordance withvarious embodiments of the present invention. As shown, apparatus 1100includes a crucible 1105 positioned on top of a crucible stand 1110within a susceptor 1115. Both the crucible 1105 and the susceptor 1115may have any suitable geometric shape, e.g., cylindrical. During atypical growth process, the semiconductor crystal 1120 is formed bycondensation of a vapor 1125 that includes or consists essentially ofthe elemental and/or compound precursors of the semiconductor crystal1120. For example, for a semiconductor crystal 1120 including orconsisting essentially of AlN, vapor 1125 may include or consistessentially of Al and N atoms and/or N₂ molecules. In preferredembodiments, the vapor 1125 arises from the sublimation of a sourcematerial 1130, which may include or consist essentially of the samematerial as semiconductor crystal 1120, only in polycrystalline form.Source material 1130 may be substantially undoped or doped with one ormore dopants, and use of a doped source material 1130 typically resultsin semiconductor crystal 1120 incorporating the dopant(s) present insource material 1130. The semiconductor crystal 1120 may form on andextend from a seed crystal 1135. (Alternatively, the semiconductorcrystal 1120 may nucleate upon and extend from a portion of the crucible1105 itself, in the manner depicted in FIG. 7.) The seed crystal 1135may be a single crystal (e.g., a polished wafer) including or consistingessentially of the same material as semiconductor crystal 1120 or may bea different material.

The crucible 1105 may include or consist essentially of one or morerefractory materials, such as tungsten, rhenium, and/or tantalumnitride. As described in the '135 patent and the '153 patent, thecrucible 1105 may have one or more surfaces (e.g., walls) configured toselectively permit the diffusion of nitrogen therethrough andselectively prevent the diffusion of aluminum therethrough.

As shown in FIG. 11, during formation of the semiconductor crystal 1120,a polycrystalline material 1140 may form at one or more locations withinthe crucible 1105 not covered by the seed crystal 1135. However, thediameter (or other radial dimension) of the semiconductor crystal 1120may expand, i.e., increase, during formation of the semiconductorcrystal 1120, thereby occluding the regions of polycrystalline material1140 from impinging vapor 1125 and substantially limiting or eveneliminating their growth. As shown in FIG. 11, the diameter of thesemiconductor crystal 1120 may expand to (or even start out at, inembodiments utilizing larger seed crystals 1135) be substantially equalto the inner diameter of the crucible 1105 (in which case no furtherlateral expansion of the semiconductor crystal 1120 may occur).

The growth of the semiconductor crystal 1120 along a growth direction1145 typically proceeds due to a relatively large axial thermal gradient(e.g., ranging from approximately 5° C./cm to approximately 100° C./cm)formed within the crucible 1105. A heating apparatus (not shown in FIG.11 for clarity), e.g., an RF heater, one or more heating coils, and/orother heating elements or furnaces, heats the susceptor 1115 (and hencethe crucible 1105) to an elevated temperature typically ranging betweenapproximately 1800° C. and approximately 2300° C. The apparatus 1100features one or more sets of top thermal shields 1150, as well as one ormore sets of bottom axial thermal shields 1155, arranged to create thelarge axial thermal gradient (by, e.g., better insulating the bottom endof crucible 1105 and the source material 1130 from heat loss than thetop end of crucible 1105 and the growing semiconductor crystal 1120).During the growth process, the susceptor 1115 (and hence the crucible1105) may be translated within the heating zone created by the heatingapparatus via a drive mechanism 1160 in order to maintain the axialthermal gradient near the surface of the growing semiconductor crystal1120. One or more pyrometers 1165 (or other characterization devicesand/or sensors) may be utilized to monitor the temperature at one ormore locations within susceptor 1115. The top thermal shields 1150and/or the bottom thermal shields 1155 may include or consist of one ormore refractory materials (e.g., tungsten), and are preferably quitethin (e.g., between approximately 0.125 mm and 0.5 mm thick).

As mentioned above, the maximum mass transfer from source material 1130and/or vapor 1125 (and therefore growth rate of semiconductor crystal1120) is typically achieved by maximizing the axial thermal gradientwithin the crucible 1105 (i.e., maximizing the temperature differencebetween the source material 1130 and the growing crystal 1120 so thatthe growing crystal 1120 has greater supersaturation). In preferredembodiments, the onset of crystal-quality deterioration (e.g., increaseddislocation density, formation of grain boundaries, and/orpolycrystalline growth) sets the approximate upper limit of thesupersaturation at a given growth temperature. For typical growthtemperatures (e.g., between approximately 2125° C. and approximately2275° C.), this upper limit of the axial temperature gradient isgenerally approximately 100° C./cm (although this maximum may depend atleast in part on the dimensions and/or shape of the growth chamber, andmay thus be larger for some systems). However, as the cross-sectionalarea of the semiconductor crystal 1120 increases (and/or for larger-areaseed crystals 1135), the probability of parasitic nucleation (on theseed crystal 1135 or in other locations) increases. Each parasiticnucleation event may lead to formation of an additional growth centerand result in grain or sub-grain formation (and thus low-quality and/orpolycrystalline material). Minimizing the probability of parasiticnucleation is preferably achieved by providing a non-zero radial thermalgradient in a direction substantially perpendicular to the growthdirection 1145 that promotes lateral growth. Formation of the radialthermal gradient also enables growth of larger, high-quality crystals athigh growth rates, as previously mentioned.

In accordance with various embodiments of the invention, the top thermalshields 1150 are also arranged to form the non-zero radial thermalgradient within crucible 1105. The radial thermal gradient is preferablylarger than 4° C./cm, e.g., ranging between 4° C./cm and 85° C./cm(although, as described above relative to the axial thermal gradient,these values may depend on the specific dimensions and/or shape of thecrucible). In preferred embodiments, the axial and radial temperaturegradients are balanced. The radial and axial thermal gradients arebalanced when the magnitudes of the gradients are within their upperlimits (as detailed below). Preferably, the ratio between the axial andradial gradients (the thermal gradient ratio) is less than 10, less than5.5, or even less than 3 at any given point inside the crucible 1105.The thermal gradient ratio is also preferably greater than 1.2, e.g.,ranging from 1.2 to 5.5. The maximum (i.e., upper limit) radialtemperature gradient is a function of the growth temperature and ispreferably defined by the onset of cracking and/or increased dislocationdensity (and/or grain-boundary formation) in semiconductor crystal 1120.At the growth temperature, dislocation arrays, or even grain boundaries,may form at elevated radial thermal gradients. Such defects usuallyexhibit center-symmetric patterns. The minimum (i.e., lower limit) ofthe radial thermal gradient is preferably set by complete lack oflateral growth of the semiconductor crystal 1120 perpendicular to thegrowth direction 1145.

As noted above, after the semiconductor crystal 1120 has laterallyexpanded to the inner dimension of the crucible 1105 the expansiongenerally ceases. However, preferred embodiments of the inventionmaintain a non-zero radial thermal gradient (which may be different fromthe radial thermal gradient during the expansion of the semiconductorcrystal 1120) even after the lateral expansion of semiconductor crystal1120 has ceased in order to maintain high crystalline quality. Thenon-zero positive (as defined herein) radial thermal gradient generallyresults in semiconductor crystal 1120 having a convex surface duringgrowth (e.g., as shown in FIG. 11). Lateral growth of semiconductorcrystal 1120 promotes growth-center coalescence, and preferably growthinitiates and proceeds from only one growth center. Even in such a case,there is preferably some non-zero magnitude of the radial gradient toprevent formation of additional growth centers. Examples of balancedaxial and radial thermal gradients for growth of semiconductor crystalhaving a diameter of approximately two inches are set forth in the tablebelow.

Growth Thermal gradient upper limit (° C./cm) Axial/Radial temperature(° C.) Axial Radial ratio 1800 25 12 2.1 2250 105 45 2.3

In preferred embodiments, the crucible 1105 has a lid 1170 withsufficient radiation transparency to enable at least partial control ofthe thermal profile within the crucible 1105 via the arrangement of thetop thermal shields 1150. Furthermore, in embodiments featuring a seedcrystal 1135, the seed crystal 1135 is typically mounted on the lid 1170prior to the growth of semiconductor crystal 1120. The lid 1170 istypically mechanically stable at the growth temperature (e.g., up toapproximately 2300° C.) and preferably substantially prevents diffusionof Al-containing vapor therethrough. Lid 1170 generally includes orconsists essentially of one or more refractory materials (e.g.,tungsten, rhenium, and/or tantalum nitride), and is preferably fairlythin (e.g., less than approximately 0.5 mm thick).

The arrangement of the top thermal shields 1150 provides control of theradial thermal profile, and hence provide the radial gradient preferredto maintain high crystal quality at high growth rates and to form andmaintain the desired thermal gradient ratio. Simultaneously, the shieldarrangements provide the necessary heat transfer to ensure the maximumgrowth rate. The balance between the axial and radial thermal gradientsmay be achieved by providing certain opening arrangements of theshields. As shown in FIG. 11, each of the top thermal shields typicallyhas an opening 1175 therethrough. The opening 1175 normally echoes thegeometry and/or symmetry of the crucible 1105 (e.g., the opening 1175may be substantially circular for a cylindrical crucible 1105). The sizeof each opening 1175 may be varied; typically, the size(s) range from aminimum of 10 mm to a maximum of approximately 5 mm (or even 2 mm) lessthan the diameter of the crucible 1105.

For example, in a preferred embodiment, five thermal shields 1150, eachhaving a diameter of 68.5 mm and an opening size (diameter) of 45 mm,are used. The thickness of each of the thermal shields 1150 is 0.125 mm,and the thermal shields 1150 are spaced approximately 7 mm from eachother. At a typical growth temperature of 2065° C., this shieldarrangement results in a radial thermal gradient (measured from thecenter of the semiconductor crystal to the inner edge of the crucible)of 27° C./cm.

As shown in FIG. 11, the top thermal shields 1150 may have openings 1175larger than any such opening present in the bottom thermal shields 1155,and/or the top thermal shields 1150 may be stacked with one or morespacings between shields that are larger than that between the variousbottom thermal shields 1155. The spacings may range betweenapproximately 1 mm and approximately 20 mm, and preferably betweenapproximately 7 mm and approximately 20 mm. Also as shown, the openings1175 in the top thermal shields 1150 may all be substantially equal toeach other. Depending upon the desired growth conditions (e.g.,pressure, temperature, crucible dimensions, distance between the seedcrystal and the source material, etc.), the number of top thermalshields 1150, the spacing between shields 1150, and/or the size of theopenings 1175 may be varied to form the desired radial thermal gradientand hence, the desired thermal gradient ratio. The radial thermalgradient may even be varied in real time during the growth ofsemiconductor crystal 1120, e.g., in response to feedback based ondetermination of the radial thermal gradient during growth. For example,the radial thermal gradient may be determined based on the temperaturesof lid 1170 and one or more sides of crucible 1115 (e.g., measured bypyrometers 1165 as shown in FIG. 11).

Similarly, although each of the top thermal shields 1150 preferably hasa thickness less than 0.5 mm, the thickness of one or more of theshields 1150 may be varied with respect to the others. For example, oneor more top thermal shields 1150 may have a thickness of approximately0.25 mm while one or more others have a thickness of approximately 0.125mm. The thickness of the top thermal shields 1150 may even be varied asa function of distance away from the lid 1170 (i.e., either increasingor decreasing). Such thermal shields 1150 having different thicknessesmay be utilized to adjust the thermal field above and within thecrucible 1115. For example, a thicker shield may be used to block moreradiative heat flow but will typically have higher thermal impact attemperatures where the heat flux is dominated by the thermalconductivity (lower temperatures, e.g. <1500°-1800°). Therefore, theresultant radial thermal gradient may vary as a function of growthtemperature, even with the same arrangement of the same top thermalshields 1150.

FIGS. 12A and 12B depict alternative arrangements of the top thermalshields 1150 for producing a radial thermal gradient within crucible1105. Specifically, the openings 1175 in the top thermal shields 1150can be varied as a function of distance away from the lid 1170. As shownin FIG. 12A, the top thermal shields 1150 may be arranged such thattheir openings 1175 increase in size with increasing distance from lid1170. FIG. 12B depicts an alternate arrangement in which the top thermalshields are arranged such that their openings 1175 decrease in size withincreasing distance from lid 1170. Of course, either of the arrangementsof FIGS. 12A and 12B may be combined with any of the other arrangementvariations described previously. In some embodiments, the arrangement ofFIG. 12A is preferred, as it increases the probability of forming andmaintaining a single growth center (where the narrowest opening islocated) at the initial stages of growth.

Having described the principles and apparatus of various embodiments ofthe present invention, the method of operation, i.e., a growth processfor AlN using the system described above is now described in conjunctionwith FIG. 13. As listed therein, in some embodiments, crystal growthinitially involves evacuating the susceptor 1115 (step 1300), e.g., topressures on the order of about 0.01 mbar (1 Pa) using a vacuum pump.The susceptor 1115 is then refilled with an inert gas or a gas includingor consisting essentially of nitrogen (step 1305). These steps arepreferably repeated one or more times to minimize oxygen and moisturecontamination (step 1310). Steps 1300-1310 may be performed byevacuating and refilling a process chamber (not shown in FIG. 11) thathouses the susceptor 1115 and various other portions of apparatus 1100,and references to the “chamber” below may refer to such a chamber or tothe susceptor 1115. The chamber is then pressurized to about 1 bar (100kPa) with nitrogen gas which is preferably mixed with a small amount ofhydrogen (step 1315). For example, a gas including or consistingessentially of about 95-100% N₂ and 0-5% H₂ is suitable in manyembodiments. In particular embodiments, a commercially-available mixtureof about 3% H₂ and 97% N₂ is employed. Polycrystalline AlN sourcematerial 1130 is placed at a proximal end of the crucible 1105 (step1320). The crucible 1105 may then be evacuated and sealed, or may beprovided with selective openings as described hereinabove. The crucible1105 is then disposed concentrically within the susceptor 1115 with itsdistal end opposite the source material 1130 (at which a seed crystal1135 may be disposed) in the high-temperature region of the heating zoneproduced by the heating apparatus (e.g., a furnace (step 1325). Thetemperature is then increased to bring the distal end of the crucible1105 to a temperature of approximately 1800° C., in particularembodiments, within about 15 minutes (step 1330). At the end of thistemperature ramp, the gas pressure is set and maintained at apredetermined super-atmospheric pressure (step 1335), and thetemperature is ramped to a final crystal-growth temperature (step 1340),e.g., in about 5 hours. As mentioned above, the final crystal-growthtemperature may range between approximately 1800° C. and approximately2300° C. During the temperature ramp, the pressure may be continuouslyadjusted, e.g., using a vent valve (not shown) to maintain it at thatfixed super-atmospheric value (step 1345). One potential advantage ofthis ramping is the enhancement of the purity of the source material1130 by permitting part of any oxygen still contained within it todiffuse out of the crucible 1105 (e.g., through the crucible walls).This diffusion occurs because the vapor pressure of the aluminumsuboxides (such as Al₂O, AlO, etc.) generated due to the presence ofoxygen in the source material 1130, is known to be higher than that ofAl over AlN for the same temperature.

Once the growth temperature is reached, the drive mechanism 1160 isactuated to move the distal end of crucible 1105 towards the distal endof the chamber, and relative to the axial thermal gradient produced atleast in part by the heating apparatus and the arrangement of the topand bottom thermal shields (step 1350). Preferably, the distal end ofcrucible 1105 is initially located within the highest-temperature regionof the susceptor 1115 at the beginning of the growth run. As thecrucible 1105 moves upwards the distal end of crucible 1105 becomescooler than the source material 1130, which promotes effective masstransport from the source material 1130 to the colder region of thecrucible 1105.

During the growth process, the pressure is preferably maintained at aconstant predetermined value (step 1355). The most appropriate value forthis pressure typically depends on the axial spacing between the sourcematerial 1130 and the (closest) surface of the growing crystal 1120, aswell as the rate of nitrogen diffusion through the crucible walls orflow through other openings. It may also be appropriate to activelyadjust the gas pressure over a relatively narrow range during crystalgrowth to compensate for any changes in the spacing between the surfaceof the sublimating source material 1130 and the growing crystal surface.

In particular embodiments, a pressure of about 18 psi has been used todemonstrate growth rates of 0.9 mm/hr with a separation between thesource material 1130 and the surface of the crystal 1120 ofapproximately 2 cm, employing tungsten crucibles fabricated by eitherchemical vapor deposition or powder metallurgy technique (such as thosedescribed in commonly assigned U.S. Pat. No. 6,719,843, the entirety ofwhich is incorporated by reference herein). Thesource-to-growing-crystal-surface distance may vary during the growthrun if the area of the growing crystal surface is different from thesurface area of the source material 1130 and the growth rate (i.e.,axial rate of movement of the crucible through the temperature gradient)may be adjusted to account for any such change. However, typically thesurface area of the source material 1130 and growing crystal surfacewill be kept nominally constant and approximately the same size so thatthe separation between the source and growing crystal surface willremain substantially constant during most of the growth.

Finally, the movement of crucible 1105 is stopped (step 1360) and acooling ramp (step 1365) is established to bring the apparatus and thecrystal 1120 to room temperature. Using pressures in the range 100 kPato 150 kPa (1 atm to 1.5 atm), single-crystal boules have been grown atan axial pushing rate ranging between about 0.4 and 0.9 mm/h, forexample, at the rate of 0.455 mm/h By adjusting the distance between thesource material and the growing crystal surface, and by adjusting theaxial and radial temperature gradients, other useful growth conditionsmay be obtained. Hence, skilled practitioners may usefully use variousembodiments of the present invention with total chamber pressures from50 kPa to 1 MPa (0.5 atm to 10 atm) and axial pushing/growth rates of0.3 to about 3 mm/h, or even higher.

By slicing or cutting the bulk single crystals of embodiments of thepresent invention, crystalline substrates, e.g., of AlN, of desiredthickness—for example, about 500 μm or 350 μm—may be produced. Thesesubstrates may then be prepared, typically by polishing, forhigh-quality epitaxial growth of appropriate layers of AlN, GaN, InNand/or their binary and tertiary alloys to form electronic andoptoelectronic devices such as UV laser diodes and high-efficiency UVLEDs. The aforementioned nitride layers may be described by the chemicalformula Al_(x)Ga_(y)In_(1-x-y)N, where 0≦x≦1 and 0≦y≦1−x.

In various embodiments, the surface preparation of crystals including orconsisting essentially of AlN enables high-quality epitaxial growth ofnitride layers on the AlN substrate. Surface damage is preferablycarefully removed in order to obtain high-quality epitaxial layersneeded for fabrication of high performance nitride semiconductordevices. One successful approach to remove surface damage from the AlNsubstrate is to employ a chemical-mechanical polishing (CMP) approach,e.g. as described in U.S. Pat. No. 7,037,838 (the '838 patent),incorporated herein by reference in its entirety. Through this approach,very high-quality epitaxial layers of Al_(x)Ga_(y)In_(1-x-y)N with lowdislocation densities may be produced using organometallic vapor phaseepitaxy (OMVPE), particularly when x exceeds 0.5. Those skilled in theart will recognize that other epitaxial growth techniques such asmolecular beam epitaxy (MBE) or hydride vapor phase epitaxy (HVPE) mayalso be successfully employed to produce high-quality epitaxial layerson the high-quality semiconductor crystals produced in accordance withembodiments of the present invention.

The growth of bulk single crystals has been described herein primarilyas being implemented by what is commonly referred to as a “sublimation”or “sublimation-recondensation” technique wherein the source vapor isproduced at least in part when, for production of AlN, crystallinesolids of AlN or other solids or liquids containing AlN, Al or N sublimepreferentially. However, the source vapor may be achieved in whole or inpart by the injection of source gases or like techniques that some wouldrefer to as “high-temperature CVD.” Also, other terms are sometimes usedto describe these and other techniques that are used to grow bulk singleAlN crystals according to this invention. Therefore, the terms“depositing,” “depositing vapor species,” and like terms will sometimesbe used herein to generally cover those techniques by which the crystalmay be grown pursuant to embodiments of this invention.

Thus, the single-crystal semiconductors fabricated using the embodimentsdescribed hereinabove may be used to produce substrates by cutting awafer or cylinder from the bulk single-crystal, preparing a surface onthe wafer or cylinder in a known manner to be receptive to an epitaxiallayer, and depositing an epitaxial layer on the surface usingconventional deposition techniques.

In particular embodiments of the invention, large, e.g. greater thanabout 25 mm in diameter, single-crystal AlN wafers are produced fromsingle-crystal AlN boules having a diameter exceeding the diameter ofthe final substrate, e.g., boules having a diameter greater than about30 mm Using this approach, after growing the boule and orienting it,e.g. by employing x-ray Laue diffraction technique, to obtain adesirable crystallographic orientation for the wafer, the boule ismechanically ground down to a cylinder having a desirable diameter andthen sliced into individual wafers, e.g., using a wire saw. In someversions of these embodiments, the boules are grown by, first, producinghigh-quality single-crystal seeds, and then using the seed crystals asnuclei to grow larger diameter single-crystal boules through acrystal-expansion growth run. Large-diameter slices from this secondcrystal growth process may then be utilized to grow large-diametercrystals without diameter expansion. In alternative versions, thecrystal growth is self-seeded, i.e. the crystal is grown withoutemploying single-crystal seeds.

In various embodiments, high-purity source material 1130 including orconsisting essentially of AlN may be produced in a crucible 1105 (orother suitable container) by reacting high-purity Al (e.g. having99.999% purity, available from Alpha Aesar of Ward Hill, Mass., USA)with high-purity N₂ gas (e.g. having 99.999% purity, available fromAwesco of Albany, N.Y., USA). In a particular embodiment, pieces ofhigh-purity AlN ceramic, e.g. weighing about 9 g or less, are placed ina bottom portion of the crucible and heated to about 2300° C. in aforming gas atmosphere in order to sublime the AlN and recondense it. Asa result, the density of the resulting ceramic may be increased toapproximately theoretical density by sublimation transport to decreasethe surface area relative to the volume of the source material. Theresulting AlN ceramic source material 1130 may have impurityconcentration of less than about 500 ppm.

In growth processes in accordance with various embodiments of theinvention, the crucible 1105 loaded with the source material 1130 may beassembled and/or disposed in the heating apparatus, e.g. high-pressurecrystal growth furnace available from Arthur D. Little, Inc.Specifically, the crucible 1105 may be placed on crucible stand 1110within the susceptor 1115. Both top thermal shields 1150 and bottomthermal shields 1155 may then be installed around the crucible 1105 withthe susceptor 1115 around the crucible 1105 and thermal shields. Thecrucible 1105 is preferably positioned such that the lid 1170 and/orseed crystal 1135 is either below or above the location of the largeaxial thermal gradient formed by the thermal shields. In the first case(i.e. below the large axial gradient) the seed crystal 1135 is initiallymaintained at a higher temperature than the source material 1130 so thatlittle or no nucleation occurs during a warm-up. If the seed crystal1135 is above the large axial gradient the initial nucleation isgenerally controlled by modification of the temperature ramp-up profile.

The growth chamber is then closed and evacuated, as described above, toreduce trace atmosphere contamination of the nucleation process and theresulting single crystal. In various embodiments, following evacuation,e.g., to less than about 1 Pa employing a mechanical Welch pump withminimum pressure of about ˜0.5 Pa, the chamber is filled with a forminggas blend of 3% H₂ and 97% N₂ to a pressure of about 100 kPa and thenevacuated again to less than 10 mTorr. This refill and pump process maybe carried out three times or more to reduce chamber contamination.Following the pump and refill processes, the chamber is filled with theforming gas to a pressure of, e.g., 117 kPa. High-purity grade gas,e.g., available from GTS-WELCO (99.999% certified), may be used tofurther ensure a clean growth chamber atmosphere.

During a ramp to the growth temperature, the pressure in the chamberincreases until the target growth pressure of, e.g., 124 kPa is reached.After reaching the operating pressure, the chamber pressure may beperiodically checked and incrementally adjusted by releasing gas fromthe chamber to a vent line in order to keep the chamber pressurebetween, e.g., 124 kPa and 125 kPa.

In some embodiments, the power supply for operating the growth apparatus1100 is an RF oscillator with a maximum power output of 75 kW at 10 kHz.The growth temperature inside the heating apparatus may be increased intwo ramp segments. For example, the first segment of the ramp may belinear for about 1.5 hours taking the top axial optical pyrometertemperature to about 1800° C. The second ramp segment may then be linearfor approximately 3.5 hours taking the top axial temperature to about2050° C. The chamber may then be maintained at growth temperature andpressure for a period of about 1 hour. Then, the crucible 1105 may bemoved up by the drive apparatus at a rate of, for example, approximately0.5 mm/hr. During the growth run, this push rate is held constant, suchthat the total travel is about 30 mm, producing a single-crystal AlNboule that reached about 35 mm in length and about 50 mm in diameter.Shorter or longer crystals may be produced by varying the traveldistance (which is directly related to the push time). The cool-downfrom growth temperature can be done linearly for the period of timebetween approximately 1 and approximately 24 hours. Once the apparatusis at room temperature, the chamber may be pumped to less than 1 Pa andbackfilled to atmospheric pressure with the forming gas, allowing thechamber to be opened and the growth crucible assembly removed from theheating apparatus for evaluation. The growth chamber may then be closedand evacuated as described above to reduce trace atmospherecontamination of the growth cell, nucleation process and resulting AlNsingle crystal.

In particular embodiments, following pump-down to less than 7 mPa, e.g.,using a turbo pump with a minimum pressure of about 0.4 mPa, the chamberis filled with a forming gas blend of 3% H₂ and 97% N₂ to a pressure ofabout 122 kPa. Following the pump and refill process, the chamber isfilled with the forming gas for the start of the growth process to apressure of 117 kPa. As described above, a high-purity grade gasavailable from GTS-WELCO (99.999% certified) may be used to furtherensure a clean growth chamber atmosphere.

During a ramp to the growth temperature, the pressure in the chamberincreases until the target growth pressure is reached. After reachingthe operating pressure, the chamber pressure may be periodically checkedand incrementally adjusted by releasing gas from the chamber to a ventline in order to keep the chamber pressure between, e.g., 124 kPa and125 kPa.

The growth temperature inside the heating apparatus and crucible may beincreased in two segments. For example, in the first segment, thetemperature is linearly increased from the room temperature to about1800° C. in 1.5 hours. Then, the second ramp segment to the final growthtemperature determined by the optical pyrometer, e.g. for 3.5 hours, maybe initiated after operator inspection.

The chamber is then maintained at the growth temperature and pressurefor a period of, for example, 1 hour. The drive apparatus 1160 thenpushes the crucible 1105 up at a rate ranging from about 0.2 to 1.0mm/hr, for example, at approximately 0.5 mm/hr. In a particular version,during the growth run, this push rate is held constant and the totaltravel is about 30 mm, producing a single crystal AlN boule that reachedabout 50 mm in diameter and 35 mm in length. Shorter or longer crystalsmay be produced by varying the distance the crucible 1105 is pushed orequivalently by varying the push time.

Following completion of the vertical travel, the vertical motion of thecrucible 1105 is stopped and the pressure is increased to 157 kPa byadding more high-purity forming gas. The power to the heating apparatusis then linearly reduced to zero, for example, in 6 hours and the systemis allowed to cool to room temperature. Following the cool down, thechamber is pumped to, e.g., less than about 1 mPa and backfilled toatmospheric pressure with forming gas. The chamber is then opened andthe growth crucible 1105 removed for evaluation.

In various embodiments, after orienting the resulting single-crystalboule, e.g., by employing the x-ray Laue diffraction technique, theboule is encased in epoxy, e.g. VALTRON available from Valtech, and thenground down to a 25-mm diameter cylinder having its longitudinal axisoriented along the desired crystallographic direction. Once the orientedcylinder is produced, it is once again examined by the x-ray Lauediffraction technique to determine precise orientation (within +/−0.2°)and then sliced with a wire saw, e.g. the Model DT480 saw, for example,the one available from Diamond Wire Technologies, into a wafer. Thoseskilled in the art of semiconductor wafer preparation will readilyrecognize that there are many alternatives for slicing the crystal usingdiamond-coated ID and OD saws. The surface of the wafer is then preparedfor epitaxial growth utilizing, for example, one or more techniquesdescribed in the '838 patent.

Seeded Growth Using Polished Semiconductor Wafers

In some embodiments, a piece of semiconductor material (e.g., includingor consisting essentially of AlN) having a known crystallographicorientation is used as a seed from which bulk material may then begrown. In a particular embodiment, a polished AlN wafer sliced from abulk crystal is employed as a seed, offering a number of benefits,including standardization and improved control over the growthdirection.

In order to grow high-quality crystals, very high temperatures, forexample exceeding 2100° C., are generally desirable. At the same time,as discussed above, high axial thermal gradients are needed to providesufficient mass transport from the source material to the seed crystal.Additionally, non-zero radial thermal gradients, resulting in thermalgradient ratios less than 10 as detailed above, are preferably utilizedto enable growth of larger crystals at faster rates while maintaininghigh crystalline quality. However, if not chosen properly, these growthconditions may result in evaporation of seed material or its totaldestruction and loss.

Preferably, the mounting technique employed in these embodiments tosecure AlN seeds entails:

(1) employing a seed holder and/or adhesive compound that issufficiently strong to secure the seed and the crystal being grown;

(2) protecting the opposite side of the seed during growth to avoidre-evaporation of the AlN, as this may result in formation of planarand/or extended void defects; and

(3) avoiding contamination of the crystal and the crucible by thematerial chosen to protect the opposite side of the seed or as theadhesive.

In some embodiments, AlN seeded bulk-crystal growth is carried out inthe crucible 1105 using a high-purity AlN source 1130. In someembodiments, the apparatus 1100 for growth of single-crystal AlN boulesincludes a crucible 1105 such as the one disclosed in U.S. Pat. No.6,719,842 (the '842 patent), incorporated herein by reference in itsentirety, consisting essentially of tungsten and fabricated by a CVDprocess. Multiple grain layers within the wall of the crucible may beobtained by interrupting the tungsten deposition several times beforethe final wall thickness is obtained. Other crucible materials may beused, such as tungsten-rhenium (W—Re) alloys; rhenium (Re); tantalummonocarbide (TaC); a mixture of Ta₂C and TaC; a mixture of Ta₂C, TaC andTa; tantalum nitride (Ta₂N); a mixture of Ta and Ta₂N; hafnium nitride(HfN); a mixture of Hf and HfN; a mixture of tungsten and tantalum(W—Ta); tungsten (W); and combinations thereof. The apparatus preferablyhouses an AlN source material 1130, for example, consisting essentiallyof high-purity AlN polycrystalline ceramic.

The tungsten crucible is placed into an inductively heated furnace, asdescribed above, so that the temperature gradient between the source1130 and the seed crystal 1135 drives vapor 1125 to move from the hotterhigh purity AlN ceramic source to the cooler seed crystal. Thetemperature at the seed interface and the temperature gradients aremonitored and carefully adjusted, if necessary, in order to nucleatehigh-quality mono-crystalline material on the seed and not destroy theAlN seed. Skilled artisans will also readily recognize that whilevarious embodiments of the present invention have been described hereinas utilizing a seed crystal to promote crystal growth, the teachingsherein may also be used for unseeded crystal growth, without departingfrom the scope and spirit of the present invention.

What is claimed is:
 1. A method for growing single-crystal aluminumnitride (AlN), the method comprising: mounting an AlN seed on a seedholder, thereby forming a seed-seed holder assembly; disposing theseed-seed holder assembly within a crystal-growth crucible; heating thecrystal-growth crucible to apply thereto (i) a radial thermal gradientof less than 50° C./cm and (ii) a vertical thermal gradient greater than1° C./cm and less than 50° C./cm; and depositing aluminum and nitrogenonto the AlN seed under conditions suitable for growing single-crystalAlN originating at the AlN seed.
 2. The method of claim 1, furthercomprising disposing AlN source material within the crystal-growthcrucible, the deposited aluminum and nitrogen evolving from the AlNsource material during heating of the crystal-growth crucible.
 3. Themethod of claim 2, wherein the AlN source material is polycrystalline.4. The method of claim 1, wherein the seed-seed holder assembly isaffixed to a lid of the crystal-growth crucible.
 5. The method of claim1, wherein mounting the AlN seed on the seed holder comprises disposinga foil between the AlN seed and the seed holder.
 6. The method of claim5, wherein the foil is substantially impervious to aluminum transport.7. The method of claim 6, wherein the foil is substantially imperviousto nitrogen.
 8. The method of claim 5, wherein the foil is substantiallyimpervious to nitrogen.
 9. The method of claim 5, wherein the foilcomprises tungsten.
 10. The method of claim 5, wherein the foil issingle-crystalline tungsten.
 11. The method of claim 5, wherein the foilcomprises aluminum.
 12. The method of claim 1, wherein the seed holderis substantially impervious to aluminum transport.
 13. The method ofclaim 1, further comprising disposing a barrier layer over at least aportion of a surface of the AlN seed.
 14. The method of claim 13,wherein the barrier layer comprises at least one of tungsten, Hf, HfN,HfC, W—Re, W—Mo, BN, Ta, TaC, TaN, Ta₂N, or carbon.
 15. The method ofclaim 13, wherein the barrier layer consists essentially of tungsten.16. The method of claim 1, wherein the AlN seed is a wafer having adiameter of at least 20 mm.
 17. The method of claim 1, wherein the grownsingle-crystal AlN has a diameter greater than 20 mm, a thicknessgreater than 0.1 mm, and an areal planar defect density≦100 cm⁻². 18.The method of claim 17, wherein the areal planar defect density is ≦1cm⁻².
 19. The method of claim 1, further comprising minimizing orsubstantially eliminating any gap between the AlN seed and the seedholder by positioning a weight on the seed-seed holder assembly.
 20. Themethod of claim 19, wherein the weight is positioned on the AlN seed.21. The method of claim 19, wherein the weight comprises tungsten. 22.The method of claim 19, further comprising removing the weight from theseed-seed holder assembly prior to depositing aluminum and nitrogen ontothe AlN seed.
 23. The method of claim 1, wherein a ratio of the verticalthermal gradient to the radial thermal gradient is less than
 10. 24. Themethod of claim 1, wherein a ratio of the vertical thermal gradient tothe radial thermal gradient is less than 5.5.
 25. The method of claim 1,wherein a ratio of the vertical thermal gradient to the radial thermalgradient is less than
 3. 26. The method of claim 1, wherein a ratio ofthe vertical thermal gradient to the radial thermal gradient is greaterthan 1.2.
 27. The method of claim 1, wherein the radial thermal gradientis larger than 4° C./cm.
 28. The method of claim 1, wherein the verticalthermal gradient is larger than 5° C./cm.
 29. The method of claim 1,wherein applying the radial thermal gradient comprises arranging aplurality of thermal shields outside the crystal-growth crucible. 30.The method of claim 29, wherein each of the thermal shields comprises arefractory material.
 31. The method of claim 29, wherein each of thethermal shields comprises tungsten.
 32. The method of claim 29, whereineach thermal shield defines an opening therethrough.
 33. The method ofclaim 32, wherein the openings of the thermal shields are substantiallyequal in size to each other.
 34. The method of claim 32, wherein theopening of each thermal shield ranges from approximately 10 mm toapproximately 2 mm less than a dimension of the growth chambersubstantially perpendicular to a growth direction along which thesingle-crystal AlN grows.
 35. The method of claim 32, wherein theopenings of at least two of the thermal shields are different in size.36. The method of claim 32, wherein a first thermal shield having afirst opening is disposed between the crucible and a second thermalshield, the second thermal shield having a second opening larger thanthe first opening.
 37. The method of claim 29, wherein at least two ofthe thermal shields have different thicknesses.
 38. The method of claim29, wherein a thickness of each of the thermal shields ranges fromapproximately 0.125 mm to approximately 0.5 mm.